SunRuanEr:
I'm worried that there's so many factors to take into account after getting an "average posting rate" that the calculated average itself would be essentially useless.
That's a fair point, that's why RPoL would have to be very transparent about how the score is created. A GM self-rating, even if intermittently adhered to, would still be nice. If I were designing it, I would answer your questions as follows.
I would include all NPC posts, both by GM and players. Whether it gets tagged to the GM or a player doesn't matter that much. The metric is supposed to show activity, not necessarily by who.
PM posts don't count. That isn't involvement of the group. Not that I have anything against using PMs as you described, but that doesn't move the group as a whole forward.
SunRuanEr:
What about the game where there's only one IC post being made a week, but there's a running conversation in the OOC that skews the average? (Or OOC PM conversations, likewise.) Would you have some way to separate the 'meat and potatoes' IC posts from the 'garnish' OOC posts?
This seems like a hypothetical problem, not real world (or virtual). Would this happen regularly in any campaign? Would a group with a massive running conversation in OOC really only post once a week IC? Regardless, OOC is not the game so I wouldn't count it.
The above are my thoughts in response to you, but I can imagine there would be a lot of disagreement on them. I still think a self-rated, searchable box by the GM on what they are aiming for would be a good addition.