There's plenty of threads on the airbag light problem at
www.renntech.org. Just try the search function. Also, this site is applicable:
http://www.geocities.com/esklut/airbag.html
The gist of the matter is the seatbelt wiring harnesses on early Boxsters are cruddy (technical term), which can cause false airbag alarm lights. Although when the alarm light is on, airbags function normally. The real fix specified in the Technical Service Bulletin is replace tin connectors with gold connectors, re-locate the grounding strap, neatly and firmly tie down the wiring harnesses, and change the the seatbelt buckles. Very expensive and I see that you are aware of this from your original post. The gambling way is swap out the offending seatbelt buckle and hope for the best. However, maybe someday in the future on a humid day, the tin connector oxidizes just a little more and bingo, another airbag light. Or, you hit a innocent little bump in the road and bingo, another airbag light. Or, maybe it never happens again while you own the car.