Would it be feasible to build a .284 based cartridge on an FN Mauser? What might be some problems?

My good friend has one in .25-284 on a commercial FN. It was built in 1969. Works beautifully. So long as your gunsmith understands the concept of properly fitting the feed rails for the fatter cartridge, you should have no trouble. Actually, it's a bit easier than reworking an FN or a '98 for a magnum cartridge.

I've seen many that feed fine with no work.