The silver stuff can now be bought from AliExpress in very small quantities in syringes, costs a few bucks delivered. I bought mine in Poland because I didn't want to wait, but the packaging leaves absolutely no doubt where it originated from
I tried it on a damaged IBM Model M membrane and it worked
Initially it has virtually no detectable conductance, I was barely able to get a reading of 100MΩ by bringing the probes within 1mm of each other. But after 24 hours of curing, resistance fell down to <3Ω, which is almost on par with the original traces on the membranes. I applied a bit too much and ended up needing to scratch the gaps between traces.
I also performed some testing on the keyboard itself: the original traces are 30~50Ω end-to-end per one sheet, and the controller reacts to a 4.7kΩ resistor applied directly between its PCB contacts (didn't try other values). So it looks like even a much sloppier repair with hundreds or thousands ohms of resistance would still do the job. Cheap graphite ink for TV remote keypads, perhaps?
Now gotta bolt mod the damn thing, put it back together and reinstall 102 hammers, springs and keycaps