No problem putting in a receptacle above the dropped ceiling. These receptacles can be used for servicing equipment located above the ceiling. You cannot, however, plug in a flexible cord for a piece of equipment that is located above the ceiling and leave it plugged in. A wireless router would be an example.
Few things are harder to put up with than the annoyance of a good example.
Pretty much...I always have problems with the HVAC folks who want to put a plug-in condensate pump above the ceiling tiles and whine that it should be okay because "it's just a tiny little load". I usually tell them that the only guarantee that comes with that statement is that it will "only start a tiny little fire".
I've seen a couple places where the FAU/condensate pump was placed above the electrical room which was hard lid (and the only thing hard lid), thus not above the suspended ceiling.
I know the receptacle itself is not a code violation in itself, but I've always been reluctant to install one above T-bar mostly because I know SOMEONE (alarm guy, HVAC, tenant, etc..) will end up using it for things not allowed.