I've looked through several saves, and it always seems to be blank... though I don't have a complete set of actual saves, just me messing around in the game. I can't think of why it would be needed, though. There are already globals for the complete planets and player choices... why would a string be needed, specifically? K_LAST_MODULE is used in K1 to record the module name you're in before you use the return to hideout/Ebon Hawk function. In that case, a string is specifically needed; numbers would be possible but it would require a lot more coding and you would have to edit it every time a new module is added to the game. But in the case of loading the correct stunt module... well, the progression is mostly linear, so there wouldn't be any point of contention.
I might have to make my own anyway, though. I realized I might want more than one.