Ken, my concern is the same as the Capn; I don't think it would be sonically problematic, but the actual fastening of the speaker could be more of a challenge than you'd like.
It doesn't seem to me like the QS T-Bracket is designed to be mounted in that orientation. At a minimum, you'd have to find a way to ensure that the speaker doesn't "slide out" of the bracket since you would not have gravity holding it in. At worst, the T-bracket or corresponding mounts on the speakers could conceivably bend or not hold the speaker flush to the ceiling.
Finally, there is a big difference in the amount of strength between "pull-out" and "shear" weight. While I have no qualms about using E-Z Anchor or similar fasteners on a vertical surface, I don't think I would hazard that method in the ceiling. So, to get the QS brackets to hold the weight, you'd need to get lucky with where the ceiling joists are, or use molly bolts or some other highly reliable (and possibly rental-ceiling-damaging) mounting hardware.
Personally, I'd find a way to do it on the wall, or if you are set on the ceiling, use a FMB (whose smaller mounting surface area should help you relative to potential damage and/or hitting the joists). Of course, if your ceiling is concrete, some of those concerns are moot.
Best solution is to just have Adam and Peter come over and hold them whenever you want to watch a movie.
