There would be a few factors that would need to be considered before doing this.
1. Is the area being used as the plenum (return air)
2. what is the structure above this area
3. How opposed is the customer to having the AP mounted on the ceiling tile
4. What is the range the AP needs to reach

There would also have to be a signed document stating that this installation isn't recommended and may diminish network connectivity, but if the customer agrees to this then why not make the customer happy. Maybe you need to install two APs to cover the same range as what the one would cover in an optimized installation.