Okay, a couple things I've thought about, or tried recently. Someone oughta get these!
1) One ring muscle up. Using both arms, only one ring. Tyson Cecka's idea. I took it to the gym @ UC Berkeley and none of the PK or gymnastics folks could get it. Some tried with hands on the bottom of the ring, others with one hand on top, one on bottom. Everyone got stuck with one elbow up and had no idea what to do next.
2) Clapping muscle ups. <--- I'm searching youTube for this right now. Bar-barians, Bartendaz... someone must have done this. EDIT:
BAM!!!! at 3:21-3:26 he does 2 consecutive MU's and claps his head! Always something to shoot for!