Also, with a wall vent in a bathroom with an exhaust fan, you would pretty much be sucking the conditioned air right out of the building and not doing much good when both are on at the same time.
Isn't it doing that anyway? I'm just moving an existing vent. Exhaust fan is already there and so is a duct, both original to the house...
The idea is just to exhaust the humid air when in use, correct?
It is doing that anyway, but when the register is located on the floor, it is also conditioning the air between the floor and the exhaust fan on it way to being sucked out of the house. When you have a register mounted high up on the wall, it can't do nearly as much conditioning. But saying that, this is a minor consideration in my opinion. For me, the biggest consideration would just be if it is possible. and there are things in the floor and wall joist cavity to block it from being done.
Also, I tend not to worry about ducting on the floor and overflowing toilets. With modern toilet designs, the bowl can hold an entire tank full of water and not overflow so overflows are something of the past unless perhaps you have kids or adults who aren't really thinking and try to flush twice. Even if there wasn't a vent there, I've never seen a bathroom floor tight enough to prevent water from leaking through to below. Generally it will run to the walls, under the baseboard, down to the subfloor and whatever is below.
