Hi Tom,
In my hand, I'm holding a Shimano Ultegra sealed BB kit. The crank-end of the cartridge is threaded (english) with a separate Al ring to secure the other side.
One way I can see that this may possibly work, economically, is to purchase readily available cartridge (Shimano 105?, etc); mill, grind or lathe-cut existing cartridge threads; machine Cr-Mo (or other strong alloy) rings that could securely locate cartridge within a range for best chainline. Is this the way Phil Wood does it?
I think there would be alot of interest, both for french and swiss, maybe italian to boot. Problem may be that a price of $50 would apply to large quantities only (hundreds, perhaps thousands) and I'm not sure if demand would warrant that. If it did, one would think current manufacturers would see the market potential. Phil Wood apparently does but not at $50. With the economy in free-fall, machine work may not be the limiting factor.
Good luck if you pursue this and I'll take one if only as a spare.
Jack Romans
Sacramento, CA