Walt wrote:
Here's the scenario:

Boat W is to windward of boat L approaching the starting line on a close
reach with her boom outboard of the gunwales in order to slow down.
Boat M comes up from clear astern of boat W and tries to go in between
boat W and boat L. At the moment the overlap begins, there is enough
space for M to pass between the hulls of boat W and L, but not enough
space for her to pass between W's boom and L. Is M entitled to room here?

L doesn't really come into the picture. This is purely a question of
M and W.

Initially, M is clear astern:

12 ON THE SAME TACK, NOT OVERLAPPED
When boats are on the same tack and not overlapped, a boat clear
astern shall keep clear of a boat clear ahead.

At some point, M becomes overlapped with W. W now has to keep clear
of M, but, M is restricted by:

15 ACQUIRING RIGHT OF WAY
When a boat acquires right of way, she shall initially give the other
boat room to keep clear, unless she acquires right of way because of
the other boat's actions.

Since there is no proper course before the start (see the definition
of Proper Course), 17.1 does not come into play. So, W needs to keep
clear of M, but she doesn't need to begin maneuvering to keep clear
until the overlap is established, and even then, she only has to
maneuver in a seamanlike manner. She's got to trim her boom back
inside the sheerline, but only needs to do so hand-over-hand on the
sheet; she doesn't need to grab all parts of a multi-part mainsheet
system and do a panic trim. She needs to head up to stay clear, but
doesn't need to throw the helm hard over.

Furthermore, if W's boom was so far out and you stuck your bow so
close in that in the course of her normal trimming of the mainsheet
the swinging boom hit your forestay, then I'd say you didn't give W
sufficient room. Probably not an issue on something like a J/24 with
a fairly short boom, but it certainly could be with a longer boom
(perhaps even one that overhangs the transom).

IOW, does "room" as cited in 18.5 include only the hulls or is it
"equipment in normal position"?

I don't see that 18.5 has anything to do with this. 18.5 talks about
a "continuing obstruction", which is generally taken to mean
the shoreline, a long dock, a fishing net, or the like. Where I sail,
there's some kind of marine construction project going on, and there's
a long line of pilings driven into the bottom near the edge of the
bay. They're spaced a couple of boatlengths apart, and the line goes
on for 1/4 mile or more; I'd call that a continuing obstruction. A
boat to leeward of you does not fit that definition.