P: (Masking) "White Balance Background"

Sample problem: You've used a daylight-balanced flash in an indoor tungsten environment. If you white balance for your foreground subject, the background color will be off, and vice-versa. Currently you can 1) select the subjet, 2) invert so that the background is selected, and then 3) "eyeball" proper white balance. But this is inexact. 

Requested feature: Within the Masking module, have a "White Balance Background" selection option that automatically selects the background (steps 1 and 2 above) and presents an eydropper tool for white balance. Much faster!