RRS 20 is an interesting rule.  It is the boat nearest the "danger" point that gets to make the call.
And whether you may agree or not, if they call for room to tack, the hailed boat has three choices: 
a - tack as soon as possible
b - hail "yout tack" and then keep clear
c - hail "protest" as she completes either "a" or "b"

See World Sailing Case 33, 54, & 113 if it applies

Case 54 may give you more insight into the timing.

Case 33 - answer 1, second sentence gives a good identication on how soon is too soon:
quote:  "...To avoid breaking rule 20.1(a), SL most not hail before *the time when she needs to begin the prosess described in rule 20 to avoid the obstruction safely*. 

before rule 20 read carfuly the definiton obstraction. than decide if the obstraction is the wind shadow area or the wharf. after it rule 20 is very simple.

 How soon is judgement on the part of the hailing boat, it wouldn't be appropriate to force them to crash tack at the obstruction, but how soon is too soon?  A wind shadow is not an Obstruction So of course the inside, hailing boat is only entitled to hail when approaching the pier. 
The answer is yes when there is a difference of opinion it goes to the room, and I'd say it'll be a tough case to win.  Document everything (seconds, boat lengths, witnesses)  so it's not a total he said-she said protest. The hailing boat, if wise, would never say it was tactical at all because that's not permitted under the rule.  I Think of Mark Room, Room to clear the obstruction in a seaman like manner. That being said Without the hailing boat or witness corroborating they tacked earlier than needed it's a hard one to win. 

Another difficult one: I sail at a couple venues where the start line is a pier, and that means the inside boat can essentially tack the entire fleet, however they have to hail in time for the boats to respond, how soon before the start can you hail for room at a continuous obstruction....

I'm not a judge, but offering my perspective as someone that's sailed in similar venues: Clipper Cove/Treasure Island, and Cityfront, both in San Francisco Bay, where boats approach a shoreline on starboard tack, and thus boats that tack to clear the shore are on port.

My perspective on your questions:

Can a boat hail for room to tack when it is (for example) still 8 or 9 boat lengths out from the obstruction or must it wait until the need to tack is more urgent?

It's significant that Rule 20 uses "soon", a measure of time, rather than "close", a measure of distance. You're talking about 8 or 9 boatlengths, a measure of *distance*, but Rule 20 uses a measure of time ("soon"). The time to cover 8 or 9 boatlengths is very very different for, say, foiling boats in breeze, versus boats in no wind.

So, I'd suggest you reframe your question to, "How many seconds in advance can someone request room?"

If I were on a jury, I'd say 3-5 seconds in advance, assuming the boat on your hip can respond.

How do you ensure that the boat on your hip can respond? One practice that works well in crowded, windy, close-to-shore venues like the San Francisco Cityfront is to say, "In 10 seconds, I WILL need room." This is a signal to the boat(s) on your hip to start to prepare for a tack so they can respond immediately once you request room. Then count down "5 - 4 - 3 - 2 - 1 - room!" This practice means that boats don't have to request searoom several seconds before they actually need it, as a hedge in case the boat on their hip is blocking them.

Now, if a boat sails into a wind shadow, they're going to slow down. Assuming boats need a 3-5 second warning, that means they'll need to get quite close to the wharf given how slowly they're moving through the water. Perhaps as close as one boatlength. (This situation is similar to our venue at Clipper Cove, where a wind shadow extends several boatlengths out from high cliffs.)

Question about the wharf: are there people fishing off of it, and if so, how far are they casting their lines? Seems quite likely that any fishing lines would go out further than 8 or 9 boatlengths, and I'm guessing they'd count as an obstruction.

Would a port rounding mark before the obstruction urge more boats to tack to the right side of the course after starting and avoid the wharf?

Philip,
At my home dinghy club where we run the races from the club house, we set an inner marker for the line and define it in the SIs as such (boats must start between the marker and the pin) the clubhouse is the "boat" but you can see how that could be problematic...  It's an old school club races solution from before we all got obsessed with the perfect W-L course and perfect start line. 

In my opinion, unless the wind shadow would cause a collision with the obstruction, hailing before necessary to avoid collision with the obstruction would validate a protest from the right of way boat. 

Craig, "... Document everything (seconds, boat lengths, witnesses)  so it's not a total he said-she said protest" 

This is great advise not only at obstructions and Rule 20's but also Rule 18.3 as well.