Man, what a nightmare you have there. And you seem to be not the only one.
I was wondering about the following point.
It should be 'quite easy' to set up a jig that replaces the programmer CPU, and-or the assigner CPU with a modern CPU. It just needs enough pins to look at each of the 8048 pins in a row. (e.g. STM32) , plus LCD interface, or serial interface.
The CPU would run a series of 'sanity checks'. If you made that open source, everybody could contribute further program parts to execute. some parts could ask the user to turn a pot, others ask for keypresses. Each part would then report for good-intermittent-missing-stuck or whatever contact it finds on that pin.
That would make troubleshooting a piece of cake compared to the hunting of the snark you are up to.
what do you think guys out there?