bwaslo asked: Does anyone know how, in fact, one would attach a speaker to a ceiling like that? Brackets at the edges?
So many options...You must gain an anchorage to the wood framing.
You should include the method of support into the cabinet design for the most aesthetically pleasing solution.
One might use plywood to span across the joists. The plywood could be the same outline as the top of the speaker...or the edges could be concealed and enclosed by edges of the speaker which were extended up.
Is the space above an attic? Do you have access to the wood framing in that area? If so, wood blocking may be added in the attic.
I have wood blocking above my R & L bedroom speakers. The brackets are made from 1/2" pipe flanges, nipples and elbows.
The speaker leads run through the pipe.
The support system is not seen from the listening position.
I use heavy wall clips on the back of the speaker to attach through a wood adapter to the pipe/flange system.
I can remove the speaker by disconnecting the leads and lifting it (1/2") off.
The speaker bwaslo is dreaming up should be able to be mounted with a hook and pivot motion. Hook one edge in a positive way to carry a significant amount of the weight, then pivot up to final position and make the connection which secures it in place. It can and should be designed so one person can do it, unassisted.