To play devils advocate, and to ignore sheering forces (or whatever the downward force from a ceiling is called)....
If you were to place the T-bar on the ceiling, well and securely mounted in whatever fashion available (stud, lag bolt, etc.), then maybe you could slide the Q's onto the bracket and then secure them with another semi-protruding nail, pin, bracket etc. so taht the speaker will be compressed onto the T-bar such that it can't slide either way (because on one side, you'll have the hanging lip and the other side, you'll have the retaining screw/nail/bracket.
I'm thinking taht those white speaker tacks which are white and come with a whole to put a nail through so you can slinky your wire aroudn a door would work fairly well. So would a snugly fitted L bracket placed against the speaker and securely into the wall.
On a side note - what kind of dispersion characteristics woudl this impart to the speaker, with the woofers playing parallel to the ceiling and the tweeters aimed downward at a 45 degree angle?
Second, what woudl this look like? How deep is the speaker box and would it look funny stickign out of the ceiling?
