^ You can do the uppercut, and it does about 1% more than b+1,1 > close/mid rune and 2% more than 2,1,4 and f+2,1,1+2. Still wouldn't really do it, though... I'd rather do b+1,1 > mid rune for the meter and better recovery time, though I think 1,1 xx Sky Drop is almost always better than both of them anyway.
I haven't really experimented much with 2,1 xx EX Skull; I always felt the meter gain and damage off 11 xx Sky Drop > guaranteed Rune Trap wasn't worth the trade off for a slightly better mix-up. You can still force the opponent to block or get hit by b+3, 2, 1, f+2, etc. after an EX Rune, so couldn't you just mix them up there with u+3 anyway once they start blocking the follow-up Quan Chi pressure? I definitely see the advantage in doing 2,1 xx EX Skull on hit-confirm when doing the Rune Trap, though.