Well, you could try that loctite stuff I posted up earlier. I have had great success with it fixing rear wheel bearings in older MX bikes whose rear hubs are too hammered to hold the bearings in. That application takes a serious beating and the loctite has held up. However, I cannot attest to how long it would hold up in a high RPM/Heat environment like a main bearing.
That being said...I think it would be worth a shot. Just glue the old bearing, put a fresh seal in and run it- you're only out the time really; if the bearing comes loose again it will be easy to pull the seal out and put it in a new case half if you get one. H*ll, even if you get another 12-20 hours of use out of it I would call it a success.