First, recommendations have always been that WAPs should be mounted high in a building rather than lower.
Second, I've mounted WAPs vertically on a wall or horizontally on a ceiling. I tend to find that ceiling mounts cover better, but coverage really is about what outside influences are there to the signal (wall construction - wood is better than metal, pipe construction - copper and iron pipes tend to really muck things up, proximity to high voltage appliances like stoves, heat pumps, etc.., proximity to microwaves - they operate on the same basic frequency as 2.4Ghz 802.11x., etc...) rather than vertical or horizontal mounting. I'm not sure how well your signal is going to deal with being below everyone, especially if there are lots of pipes or wires below the floor too.
When I'm in a building with difficult access to the ceiling I tend to find a closet where I can run cable through to the ceiling to conceal the wire and then mount it like a smoke detector. With PoE you don't have to worry about power to the unit, just the Ethernet cable so that makes it a lot easier.
Good luck.
