Question on installing a tub

I'm putting a drop-in tub in a remodeled bathroom. The drain and water supply lines will be new. The problem is I have no access to directly under the tub once it's in so how do I connect it with the drain?

The layout is thus. The bathroom is on the second floor and I have access to the space between floors so I can run the drain pipe and supply pipes through it to a spot under the tub. I can then cut a hole in the floor, where the tub will be, and strap the tube to subfloor and cement on a 90 or a trap. At this point if I drop the tub in I have no way to by-hand connect the tub to the new drain.. what do the pros do here?

The supply should be easier because the drop-in tub sits in a frame which won't be enclosed yet so I can make those connections.

Thanks for ANY Advice,
BlackNoir

I think I just found the answer. I don't have the tub yet, going to go look at one tomorrow, but is it common for the drain for the tub to go directly into a horizontal pipe which runs along the bottom of the tub and connects to the overflow, thus putting the drain pipe on the side of the tub instead of under it? I thought the drain would go straight down and thus be surrounded by a mortar bed but if not I can just get to it via the access panel for my supply lines!

Please tell me this is the common technique.. :)

It is and make sure your tub drain connects to the overflow before the p-trap.