Rule 86.1(c) allows a class rule to change RRS 49.  

Good question Thomas. I've been wondering this too from a non class racing point of view.

My interpretation is that a trapeze boat racing in an open class, not subject to class rules, can't use trapezes unless the SI's specifically allow it? Yes/No?

In general, if you are racing in PHRF or similar, handicap racing the boat is expected to be sailed/equipped in accordance with the class rules.  That is often something that is a specified as a part of the handicap rule.

Thanks John. 
2 Q’s
1, is that to comply with the phrf rules, ie, to keep even with your handicap, or to comply with the RRS?
2, not all boats have a class, or class rules, what then?

1. Both really. RRS 78 requires a boat to be maintained to her class rules, and AIUI most dinghy handicap racing worldwide is conducted under empirical handicap systems of the Portsmouth Yardstick style where handicaps are allocated to classes, so if a boat is not maintained to class rules then, at least here in the UK, a handicap published for that class is not valid for that boat. 

2. Is an interesting one. Speaking utterly personally, not with my RYA Empirical Handicap Advisory Group hat on, I suggest there is no requirement in RRS for class rules to be issued by a current, or indeed any Class Association. I have issued 'class rules' in my own name for a one off dinghy I designed myself, and manufacturers commonly write class rules for new designs before a class association is formed. I am pretty sure that the RYA has published Portsmouth numbers for classes that no longer have a functioning CA. 

So it would seem any class rules existing in some form would be valid. RYA Portsmouth Yardstick seeks to include every boat, no matter what their class status is, but equally its vital that race organisers know the configuration of every entry in order to allocate as fair a handicap as they can.  I have received guidance in the past from RYA rules folk suggesting that if a boat is not maintained to its class rules it ceases to be a member of that class. To enter a boat claiming it to be a member of a class knowing that it does not comply with class rules is a clear Fair Sailing Issue. But is a statement by the owner on the lines of 'My boat is an xyz with a trapeze added. This changes RRS 49. ' sufficient to comply with RRS as a class rule? In my personal opinion that would be acceptable for UK Portsmouth Yardstick purposes. 

Thank's for the comments. Most class rules change rule 49 to allow trapezes, but unless there are class rules changing 49.1 I doubt that using a trapeze is possible. 

This is up to the handicap system to sort out if the boats are not competing One Design. 

For instance, this is from ORC …

Here, as with PHRF, sport boats are a separate class within the handicap system and they anticipate the possibility of trapeze. 

Later in the base ORC rules, section 2 details the RRS’s that are changed.  Here are the crew weight rules. 

“200 Crew Weight and Position

When you go to the ORC Sportboat Class doc, you find that even there the sport boats are separated into 3 divisions.

Here are the ORC Sportboat rules (only 3 pgs) along with the trapeze calc and Sportboat class splits. 

PS…

The reason I pointed out ORC is to show that 

It’s a tall-order to attempt to have fair competition when mixing apples and oranges. 

@Jim,
Would you be willing to share your self made class rules?  I have a one off that in addition to getting stats into the RYA PY systems I could probably improve what I have done with a one boat class rule published.

http://devboats.co.uk/sailing/plusplus/hullrule.htm They aren't ERS compatible though. My recollection is that at the time I reckoned ERS was not well adapted to box rule boats. There are more classes with ERS compliant rule sets to look at now, and better examples frameworks in ERS. With none ERS rules I think the current international Canoe rules are superior to my rule set. In particular its a better rise of floor rule I reckon. 
You're very welcome to contact me off forum if you want to discuss this more. The email link on the plusplus pages seems to be broken (must fix that) but if you go higher up the tree you should find a link that works. 

Thank you, I think that is more than enough and much appreciated.  I hadn't considered this idea.  I did get a few other people to modify a boat once to create what was a new class that only sailed at our club, I considered doing something like this to help get the rating recognized.

Was that for a dinghy? You're using RYA PYS in the US now aren't you? You're unlikely to get a published number for a one club class, but there's also the facility in it to produce club numbers which should help you. It's also not rocket science to start with a few headline dimensions and make an estimate by interpolating between similar classes to get a usable trial number. Again you're welcome to contact me off forum to discuss. 

So far I have not yet encountered any area on in the Mid-Atlantic or Southeast US that is using RYA PYS.  I have registered our club after some drawn out internal discussions about the US D-PN chart having not been updated in two decades, but am still awaiting RYA to approve our request for an account.  I was just at a southern event and it seems the PRO will never move away from the abandoned D-PN list, which means that new boats cannot compete at his club.

Appreciate the offer and if I run into complications I'll definitely remember this and contact you.  I think this will be enough to at least start and since I'm a scorer at our club.  I need to make sure I can show the math so that others can assist with adjustments so that my boat is not being unfairly advantaged.

I understand that a boats handicap will need to be reviewed / adjusted if trapezes are added (or any other change made, weather a boat is of a class with class rules, or not, got that. But I don’t understand how it becomes a handicap systems job or ability to approve overruling the RRS.
Rule 49.2 for example makes an allowance for class rules to change the rule:
‘When lifelines are required by the class rules or any other rule, competitors shall not position any part of their torsos outside them…..’

Rule 49.1 does not
‘Competitors shall use no device designed to position their bodies outboard, other than hiking straps and stiffeners worn under the thighs.‘

If I can notify the handicap committee for club handicap, or the administrator of a national / international system that I am going to ignore other RRS, does that mean I can, or can they approve that to be legal? Say RRS51 through RRS55. 

There are handicap systems that are described as class rules and make alterations as per 86.1(c). IRC is one.
RRS 86 and 87 are the important rules for determining what can be changed, its not generally embedded in the individual rules.