I too, have wondered about blending late additions of hops with greatly varying AA%. Shooting from the hip, I've been scaling my late additions according to AA of the hops, thinking this will bring balance and allow the unique character of each hop to come through.
For example, if I had an ounce of Columbus @14% AA at 10 mins, then I would need to add ~2.5 ounces of Cascades @ 5.6% AA to bring out a distinct Cascade character because it's AA is 2/3rds less than the Columbus. Although as I type this out, I guess measurable bitterness from the AA and the flavor\aroma characteristics are two different things and probably not related in a linear way.
Anyone doing it this way or have some experiments they've tried to see if this is case?