I think I am facing a similar problem with my bathtub drain. I'm building a new garage with lab above. The lab has a bathroom and the floor (garage ceiling) is constructed of I-joists. The walls are all up and largely finished and everything is perfect...except:
As I placed the tub, which is inches from an I-joist, I couldn't match the bathtub drain with the main drain line input, which is on the other side of the I-joist. If I could cut the top of the I-joist, no problem. But structurally, I can't cut the top of the I-joist and the main drain is so long that I can't lower the main drain and keep the slope proper while maintaining garage headroom. (I refuse to put a post in the garage to support a break in the joists.)
I can't lower the tub.
I tried a flexible plastic pipe to match the roughly 3/4" vertical mismatch between the two pipes, but that has broken with use.
I don't see a solution other than an extender on the bottom of the tub to lower the drain flange and the horizontal drain pipe or a vertical discharge drain flange. Does either such animal exist?
Does a metal flexible drain pipe exist?
Any other ideas?