This situation is defined by what rules don't apply vs rules that do apply.

Rules that don't apply - 18 (Marks) and 19 (Obstructions).

Basically, the leeward boat can luff as much as she wants (up to head to wind - there is no proper course before the start) and the windward boat must keep clear.

Leeward boat is only limited by Rule 14 (avoiding contact) and 16 (allow the windward boat an opportunity to keep clear).

On a practical matter, a lot of "comminication" by the leeward boat will cut down on the traffic:
"Don't go in there! You're barging & I'll take you out!"

Real experience when starting at the committee boat in a big fleet - patience is your friend. Often the guy making the most noise (head to wind right behind the signal boat) will bear off with about 10 seconds to go and leave a boat-sized hole right at the committee boat. By hanging back and a little above the noisemaker, you can pounce on the hole.

It all depends on his ability to get his boat moving at the right time though. If he gets caught in irons, you're screwed. That's why starting at the committee boat end is such a low percentage maneuver. I only do it when the boat end is heavilly favored (bad RC work or really shifty wind) or the right side is so favored, I want to tack almost immediately after starting.