Joe, I know that propane is dangerous, but incompatible with the lubricant oil? Propane should be compatible with any of the R12 systems out there. R12 systems should be able to use propane in a direct switch, although R134a systems sure wouldn't work. Also, the seals should be fine as well. Are you talking about running propane in automobile R12 systems? Those older systems have flexible hoses which aren't leakproof. That's why you needed to have a springtime refresh of your refrigerant.
Also, the performance drop from R12 to propane should be around 10% (not temperature, but heat moving capacity).
A typical homebrew setup should use less than 1.5 oz of propane, which is less dangerous than a 220VAC outlet or a natural gas stovetop. If you silver solder the copper joints and then run a leak test, the system should last darn near forever before springing a leak. You'll lose a compressor and need to evacuate and repair the loop before any propane escapes.
Correct me if I'm wrong, but propane really shouldn't be that big of a deal if you don't try to torch a charged system. However, I'm still laughing at using a torch to "test" an actively running system with propane in it. Open up a solder seam by accident and see the torch flame suddenly get MUCH bigger....
Where's Darwin when you need him?