It depends on the material for how the radio may be effected, but I do think all MR's are plenum rated, basically meaning they can be installed in the ceiling without causing a fire hazard.
For safety, I would be sure to mount it securely so it can't fall on someone's head if they open a ceiling tile, for example.
I once had to install ~200 APs of another manufacturer above the ceiling grid for "architectural reasons" (they were ugly). That reminds me of another suggestion- label/mark the ceiling where the AP's are placed so you can find them again 😉
I have done this before with MR16s, MR18s and MR20s. It works fine. Access points in general are somewhat directional so with them pointing at the floor you probably will not get the best coverage, however if it is your only option and you have enough points to cover the area needed then it will work.