No, you CAN NOT fit racing seats in the back, custom made or otherwise. The design of the car WILL NOT allow it.
That is of course assuming you don't want to drop the differential and convert the car to FWD, relocate the gas tank to the differentials former location, relocate the airbag control unit that is housed under the rear seat and recalibrate it for its new location, and have a completely new floor built to allow for actual clearance the seats need, then stetching the wheelbase (and therefore relocating the rear suspension setup as well) to allow the new floor in place without coming in contact with the rear wheels and reworking the exhuast piping so it accomodates the new underbody shape, plus adding all sorts of body work to both allow the wheels to fit where the bumper was and to coverup the hole created by where the wheels were. I suppose doing all that would make it possible.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>