I'm considering making an Apple-1 replica, and so I've looked into the availability of 256x4 bipolar PROMs like the MMI 6301.
I may or may not succeed in finding blank ones (and finding or making a programmer), but here are a few blue-sky ideas about taking PROMs that have already been programmed and refurbishing them, that is, rewriting them with new data. Take them with a grain of salt. :-)
1. First blow all the fuses with a programmer, then use a FIB machine to selectively deposit metal at the fuses to reconnect a subset of them.
2. Same idea, but make a mask with holes in it, then sputter metal through this mask onto the fuses
3. Use a laser welder to reconnect fuses (this may or may not work depending on the geometry of the blown fuse)
These all require decapping the chip, which is simpler with a ceramic one. I think a ceramic sandwich can be re-fritted to regain the hermeticity; but if you're willing to give up hermeticity, then something simple like epoxy would work. Also, I'm not sure if there is passivation over the fuses, but at least FIB can drill through the glass if needed.
These techniques are a stretch at the hobbyist level, but a commercial chip-reverse-engineering shop should be able to execute them: it's just a matter of cost. Or, perhaps, a university group with the appropriate equipment could be coaxed into taking on the task.
Best not to throw away any programmed 6301s, just in case.