I just have an idea with a large forum but dont know how to code it for you. Just post it here to share my idea, collect your opinion and some of you will help all of us:
A mod that count (just count only) total Private Messages in a period of time and return as a result of a function. This collect number as SMF collect number of views, posts, topics and users per month or per year at the of statistic page.
This is for fun with a large forum, not for you with small and medium because user may see: "Ahhh, this month is no pm, last month 9, before of last month 3, ... :D"