RealGM

Gives the user stats on not the player at question, but the stats of the player whom he was guarding? Like points allowed per game vs matchup or like FG% of match up etc.

82games.com is your best bet. But I don't think so. they do have a "suggestions" email address....

These guys have that kind of data, but you gotta pay.
http://www.synergysportstech.com/products.html

I'm gonna send this to the stats board, in case anyone there knows about this.

82games.com has that in the opponent counterpart stats on the "By Position" tab. It is per 48 minutes and a guy may not always be truly covering his positional counterpart, but it gives a pretty good measure of what you are looking for.