I've already got an aftermarket FPR, which is what led to my routing difficulties. Since the car was already apart for some other work I went ahead and tried it -- as far as I can tell, the ports on the rail itself are the same for feed and return. All the other lines stayed the same, just the fuel is coming in on the opposite side of the rail now.
The car is still disassembled at the moment, but I'll post an update here once I get it all put back together and see if there's anything funky with the fuel lines!