Gaius,
I've never seen a 12S but from the photos on the web it looks like there's a good chance they will work. The boards and their connector patterns look the same as more current ones. The spacing from the left side (looking from the front of the preamp) looks identical to that for my boards.
It appears that the phono boards are in back, the output amps are in the middle and the buffers are toward the front. You would use the time-aligned filter and remote control boards in the front 2 slots (doesn't matter which one goes in which slot). Since they are the same length to the left of the connectors as the originals, they should fit, with more sticking out to the right.
One thing I noticed in a review of the 12S is that it used a 30v power supply, instead of the 25v supply for the 32 and later NACs. While all the parts are rated OK for that higher voltage, I would recommend using a regular HiCap if you can (and the connector fits) to reduce the heat load on a few of the parts. Not a big concern, however.
Good luck!
Kit