If the fixtures are being custom manufactured anyway, they why not make the 'tail' with conductors suitable as circuit conductors? They can as easily build the fixture with 12ga stranded THHN conductors as 18ga...and then you have no problems with the tail in conduit. They could install a 100 foot tail and you could pull the conductors all the way back to the panel without a problem
This doesn't address any sort of support issues, but I don't know that there really is a problem.