The only time the pilot pools mix is draft and reserve. Technically you could have a RSV or drf trip on the 75 consisting of a 75 capt and 76 fo, with the 76 guy getting wb pay and the capt getting narrow body. And vice versa. You could have a drf 76 fo getting paid more than a RSV 75 capt. But that can happen anyway on the straight wb.

Personally, other than all airplanes at wb rate like ups, this seems like a pretty good solution.