I would venture to say ridiculously small.
Considering most flights are not worthy of mileage running, and then figure the routes that are worthy of it are still mostly filled with average customers the % would be super small.

I would say the % goes up:On weekends, in december, and on super sale flights such as the GRU fare last year, or the LIMA fare a couple weeks ago.

I dont know anyone who mileages runs other than on this site. I do know people that will take an extra connection intentionally for a few extra segs on a flight they are already taking but that is it.
Truthfully I have never actually done a MR. The most I have done is taken 2 weekend trips from PHL to the PDX and SAN for a weekend that was more or less cheap entertainment for the weekend.