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.