My plan is to look at on-wall M3s and find a way to secure them to the ceiling as I too do not want to cut "in-ceiling" sized holes in my soundproofed ceiling.
That is as far as I have gotten with the idea... I don't know if I trust the wall bracket to 1) hold its bracket shape from the force of the speaker pulling away from the bracket vs. just "down" on the bracket, 2) have enough length to the bracket to have the speaker vibrate off of it, 3) be secured enough to the ceiling with just screws since they have more strength on a wall pulling down at 90 degrees than on a ceiling pulling straight down.
Sure for #3 you could look at toggle bolts or something, but that isn't the biggest problem.
Without rigging something up, I just don't know the answers to the other questions.
