[SiriusHardware]

As things stand there are actually two or three GPIO pins currently unused so it could stretch to reading an 8K or 16K device.

With regard to relying on diode clamps, I think the lowest series resistors I used were 2K2 all the way up to several hundreds of K but in all cases the behaviour was as though there was no diode clamp action occurring on the Pi's inputs. So if resistors are used, they do indeed need to be in the form of a divider rather than just a simple current limit.

As mentioned in one of the other threads, Arduino does have a 'native' library for communicating with SD cards so that would be one way to get the code read out into a 'firm' form for backing up or for comparison with the original code, if available online.

I've also seen a third-party Arduino library for facilitating Xmodem / Ymodem transfer of binary files to / from any host running a terminal program which supports those protocols, such as Hyperterminal.

With the Pi having a filesystem right from the get-go with no extra hardware needed, that aspect is slightly easier to handle with a Pi than it is with an Arduino.

One thing I haven't really investigated is the maximum current available from the +5V output on the Pi, my Pi 2B runs a 2732 OK but the older PROM devices tend to run warm so their supply current may actually approach or exceed that available from the +5V output. In this case what you might have to do is take the +5V supply for the PROM from the + terminal of one of the USB sockets on the Pi rather than from the +5V pin on the GPIO connector.