Is a conduit up out of the floor supporting a box with a receptacle an acceptable installation?
(for a piece of kitchen equipment out in the middle of the floor (not near a wall))

Sure, you can do it. BUT ....

Be aware that the thing will have the living daylights beat out of it as the tables get shifted about and the floor gets hosed off. Plan accordingly.

Which means ... 3/4 rigid stub-ups, a second support to keep the box from twisting, compression connectors to allow future box replacement, and cast-iron FS-type boxes (not the wimpy Bell boxes). AND have the GFCI somewhere else.

Normally pendent drops are used.

The Health Inspector will normally be your objecting party.

cgw:

I agree with Reno & Tesla. I would also add...check with your local AHJ first.

In the fast food business I saw a mix of the FS boxes Reno described on 3/4" RMC stub ups and pendants.
They really try to put in a stem wall with regular receptacles on them and FRP sheathing or tile for the wash down. I imagine with the GFCI requirement the pendants are even more attractive.

If you're using GRC and cast boxes, you'll have to use Erickson couplings if you want to make any future box replacement easier. Also, the permanent location of the box will interfere with any future rearrangement of the Kitchen.
I vote for using drops like you see at all of the McDonalds.