Schluter, the makers of Kerdi, do offer a tub drain. Last I heard, they were essentially hand built and not inexpensive, but they do make them. You may need to call them, as I'm not sure they put it in their catalog on-line.
If I was going to do this, I think I'd use Wedi. They make a tub kit. If it doesn't have the shape or size you want, they can make one for you (not cheap, but then custom labor isn't either). Being made of high density foam with a special coating to allow thinset to stick, it would warm up quicker and hold that heat longer than one made with more conventional deckmud methods AND, you'd have an engineered system that would be both structurally sound and have a guarantee. Basically, thinset it down, urethane the pieces together, and start tiling...a huge time saver. Once the area was prepped for plumbing, you could probably have it done, including grout (if you used a quick setting mortar) in one long day. You'd take days to do a conventional one (this assumes a pro is doing it...quick setting mortar is not recommended for those faint of heart or inexperienced!).
Kerdi could be used, but it would be harder to make it conform to curves, unless you wanted flat surfaces (no compound curves). Course, tiling compound curves isn't the easiest thing to do, either, if you want it to look good.