Further to the problem 'bad cable', I went into a little further...
I removed the case from the SID USB cable, and found that one of the resistors was missing from the circuit board. One end of it had a cold solder joint when it was assembled, and had most likely fallen off when the board was originally manufactured - before the case was even put on and sealed, making it never able to communicate with any module. The resistor was not inside the waterproof-sealed case, so it has never been there. (The person I bought it from never connected it to their PC, so it just sat waiting to surprise me
)
Well, a new resistor soldered in place, and boom -- success
I'm not suggesting this is a wide-spread problem, and probably my SID is likely the only one ever to suffer this specific missing resistor problem -- but it's something at least someone else might find while googling as a possibility for this similar symptom of having the USB device work, but zero communications with any modules.
Finally I would actually recommend not trying this fix -- first off, you could fry your computer or RK system quite easily if you do it wrong (you have been warned) -- and second it would be very easy to destroy your SID during the disassembly process. I have much experience with small electronics, components, and surface-mount soldering - and went into the project understanding full well that I was likely to end up with a useless USB device afterwards and was OK to just throw it out when done