I have installed SPD cleats on a pair of old stiff soled hiking boots and they worked rather well. The biggest problem is that they weren't insulated and too small to wear many socks.

I would say go with it. It sounds like a good idea. Cross country boots are usually insulated to some degree and the sole is somewhat stiff on some types of binding models. Also, on the kind that I have the sole has recesses to mesh with the plate on the ski to keep it aligned. You might be able to build up some epoxy resin and fiberglass in this spot to make the mounting plateform for the cleats and also make the sole under the ball of the foot stiffer.

The 3 pin type have softer soles and may not be as stiff but probably easier to mount a cleat to since the sole is more flat. You could make a plate out of aluminum sheet and glue it to the sole for a better cleat mounting area. I think this could work rather well. Also, there are probably lots of old cross country ski boots around that you could get for next to nothing.

Based on the success I had with the hiking boots I say give it a try. You could use some oversized boots for really cold with more socks.

One thing tht you will want to consider is that there needs to be some kind of raised up area around the cleat that allows you to stand up and walk around without falling. In winter on ice this is really important.