The possibility of a P2 tag was raised a while ago, around the time that P2 came out.

As I recall, that was declined on the grounds that there wasn't the volume of P2 threads to support a separate tag. In the year or so since, I can't recall more than a handful of P2 threads in the D&D 3e/3.5/d20 subforum. I think those who like and play P2 are having most of their discussions on the Paizo boards.