I don't think this is feasible. The FPGA is actually the part that is most tricky to solder onto such a board (and easiest to mess up), so it seems backward to me to ship kits without FPGAs.
Yeah now that I think about it I would probably include the FPGAs but there would be no price discount on them. A kit like this isnt meant for someone with no experience doing surface mount soldering. Someone who has experience doing BGA reworks and repairs would have no problem soldering these FPGAs. I don't see any problem in allowing people to get discounted boards by assembling them themselves. Of course we would put a big disclaimer warning people that soldering these small parts and BGAs is not easy and should be done by someone with experience and we offer no warranty at all on the kits.