Hi Mike,
It sounds like the nature of the modifications you are proposing will be difficult without very specific knowledge about the board implementation (as you suggested). With that said, I don't think NI will be willing to provide such information it may still be considered proprietary even for old HW products. In any event, how difficult do you think it would be to simply write new code to perform the calibration? It shouldn't be too difficult with the command set from the instrument user manual and your favorite programming language!
If you have the option to simply implement the calibration code, I would suggest trying to go that route as it may be faster than tracking down the information you may need to further pursue your current thoughts.
Thanks
and if you have programming question related to GPIB I/O definitely repost!
JLS