Sounds like it's a little more complicated than the E36. Like I said, I haven't tackled the E39 yet so thanks for the advice.
I still think you need to remove the bottom seat cushions to keep the SRS sensor on the passenger side. Unless there is a sensor on both sides....there was just a pass side sensor in the E36.
The thick hog pins are exactly why I took the seat bottoms to a trim shop.....they quickly removed and re-installed the covers.