So it's the casein that concerns you, or the fat-free milk?
I've read that protein fluff works best with pure casein, as the other types don't actually "fluff." That doesn't mean they aren't still delicious, but it will be a pudding-like consistency as opposed to Cool Whip.
If you wanted to try mixing casein with egg whites, though, that might work. Just save the yolks for breakfast!
5'6" 27 y/o female, Primal 6 months and counting.
CW: 160 -- in single-digit dress sizes for the first time in 12 years!
GW: 150 -- for swimsuit season