I do not believe that there is anything in the rules that requires either end of the starting line to be anchored. (Clearly a mark-bot is not anchored.) What is required is that the marks remain on station within reason. We have all seen situations where the lee created by a boat passing an anchored windward mark causes the mark to move to weather. In that situation, it is "within reason" that the mark was on station. 

In your example there are two potential issues: First, that the mark-bot could be unreasonably off station. Second, that an unanchored RC boat who was helping determine OCS boats on the line might not be able to see the line accurately. In both cases, a request for redress could be made if a competitor felt that the RC had made an error. But, having an RC boat not anchored while siting the line probably would not be persuasive. Many RC boats are perfectly capable of maintaining position without anchoring. Therefore, evidence in the redress hearing would have to show that the RC boat was in a place where they were not capable of siting the line accurately and that the out of position RC boat was the one actually making the OCS call and not some other call for the PRO.

Finally, I have used an RC boat to windward of the line stationed mid-line to spot sail numbers for me. This is critical in a large fleet. (EG: ILCA) 

Todd, it's been said that the RRS is an "open rule set", such that there is broad lattitude in what is allowed unless it is specifically restricted or contrary to what is dictated with a "shall". 

No rule requires the pin end vessel to be anchored.

WS Race Management Manual

M2.2 The Pin end says

When a buoy is used then the person sighting the line has to anchor his vessel on the extension of the start line, lining the Pin end buoy with the mast on the committee vessel. When anchoring he must leave sufficient room between his vessel ad the buoy so that a boat may pass between the Pin end vessel and the Pin end buoy when flag I has been displayed.

This is not a rule.  The RMM indicates good practice.

Case 136 says

 

In finding facts, a protest committee will be governed by the weight of evidence. In general, a race committee member sighting the starting line is better placed than any competing boat to decide whether a boat was over the line at the starting signal and, if so, whether she returned and started correctly. 

So, if you are unhappy about a call, you would have to convince the protest committee that:

Thanks all, solid answers.

If the pin end is a bot, the outside line caller must necessarily NOT be anchored, because a bot is in constant motion relative to the anchored signal boat.

On this point I would have to disagree. The bots on "anchor mode" tend to very stationary. 

Correction: The bots are stationary, but the signal boat responds to shifts in the wind and current.
Hence the pin is in constant RELATIVE motion to the signal boat and the spotter must be mobile enough to range on the line.
Here in the Pacific Northwest, where the anchor rodes are in hundreds of feet, that is a significant issue.

This is not directly to the Original Question posed, but Markset Bots are not stationary. Indeed, competitors have developed a name for it: “Botted”. By that they mean that wind and tide (especially in a place like San Francisco Bay where the wind at 28k and the flood tide running with the wind frequently reaches 2.7k) will push a Markset Bot many feet to leeward during a puff. As a boat attempts to round that mark, if the puff abates, Markset Bots have been observed charging many feet to windward and making contact with a competitor who was on a layline to pass to windward only a few seconds early. That competitor has been “Botted”. Protest Committees are starting to hear about this and competitor are rightfully unhappy.

This is not a new problem and is not limited to Markset Bots. Anchored marks and boats all move around with the wind and current. Indeed, we often see a tide line move across the starting line putting the pin end in one current and the Boat end in another. Generally, this results in an AP being displayed and the RC sorting this out before restarting the sequence if the effect is pronouced. This is a primary reason that when acting as PRO I prefer an un-anchored pin end RC boat, so  it can keep itself aligned with the extension of the starting line; or, have the pin end actually be the RC Boat so there is no doubt about sighting the line.

Fortunately, modern technology may solve this problem in the near future with accurate locations for the marks and the boats on the starting line displayed and OCS boats identified electronically. (But that’s an entirely different discussion.)

In my experience on an anchored pin vessel with a relatively long open cockpit, I can get quite a large range of movement, fore and aft to keep my eyeline on the transit.  If my coxwain is on the ball, he can also respond to wind or tide driven swings by adjusting the anchor line.  Admittedly I've never had to do this in big seas with big scends.

I agree with Beau that making the pin vessel the  starting mark goes a long way to solving the problem.

Was the boat assisting in ID of boats spotted from RC end perhaps?