Does anyone know if it's theoretically possible to read the eeprom of a CS20m, for instance, to obtain the original patch programming when all this information has been lost?
The CS20m dates back to 1979. Any data written to memory that long ago is surely lost, or at least corrupted (my guess). It had 8 memory locations that memorized current settings - even if you were able to extract data, there would be no way to know it was the same as when stored. Certainly the battery responsible for keeping the data backed up has long ago lost its charge. (I'm assuming a battery of some kind was used back then).
You may want to contact Yamaha Customer Support (Service department) they may have information on your options (if any).