Thanks Dualist! I've been waiting a while to have everything set up for him to ride and bust out this shirt.
Geetee, you need to either get a deactivation bar and modify a 944 buckle (which I did, can then use any car seat you like), use the same bar with a Porsche child seat with built in buckle, or buy the switch from eBay user hfaning.
Any of these routes, though, you need to get it activated by a dealer or Indy with the proper Porsche diagnostic stuff (durametric will not do). Here's the service bulletin for them to follow, seems to be somewhat hard to find:
http://www.pelicanparts.com/techarticles/Boxster_Tech/96-MISC-Child_Seat/tsb.6923.deactivationbar.pdf
I doubt you need paperwork. When you buckle into the bar the airbag light flashes for 30 seconds and goes away, I assume that would be some form of proof if it was ever an issue.