Good question. I teach this stuff and find it one of the most confusing
parts of the rules. Remember it doesn't matter what we think is right, but
what the rules say as that is what the Coast Guard and the courts go by.
(Remember, you can't use the words "Doesn't make sense" and "Government" in
the same sentence).
Anyway:
There is no mention of being hove-to in the COLREGS. Basically a boat is
considered underway unless it is anchored or moored. It is interesting to
note that in some maritime cases even a boat that is anchored, but with the
anchor dragging, has been ruled to be underway. The anchor light can only be
used when made-fast to the ground. A boat doesn't have to be making any
headway to be considered underway (a boat that is hove-to is moving through
the water -- maybe not in the direction one wants). So by these definitions
the proper light configuration for hove-to would be as is you were sailing.
Also this would require that a proper watch be kept, even if hove-to. There
are no provisions for a boat underway not to have a watch (in 1984 a court
ruled against a single-hander who took a 30 minute nap and had a collision).
To confuse more things there is the provision called a "vessel not under
command." This would be a vessel that is broken down for whatever reason and
can't maneuver (not that the crew doesn't want to maneuver (e.g hove-to).
Even a sailboat without an engine that is totally becalmed could be
considered not under command. The light pattern for this vessel would be two
vertical all around red lights and, if making any way, the side and stern
lights. This is rule 27 of the COLREGS.
That is my take on it.
Chris