On The Muppet Show, there were only a few shots of the audience tapped, reused whenever a shot of the audience was needed. In some cases, they are shown singing to whatever song is being performed (maybe it's just me but the lip-synch doesn't look too bad regardless of the song). Sometimes, a shot of the audience would only be used once, if it had something to do with the act being followed (such as when Fozzie had a tough time with the whole audience).
But in the Madeline Kahn episode, following Happy Feet, the audience shot is a shot not used anywhere else. I noticed Zelda Rose wasn't in the audience, and I don't think many of the other same characters are, either. But I wonder if that was meant to make a big deal out of Kermit's tap-dancing performance. Another thing I noticed about that: Before cutting to the audience, we briefly see Statler and Waldorf cheering, not really making any funny comments or insults or anything.
I recently watched the "Sherlock Holmes" sketch on YouTube, and saw that it's actually 2 minutes and 14 second slong (well, the YouTube clip ends with the first second of Kermit introducing the closing number). I guess part of that had to do with the fact that the sketch wasn't intended on being the UK spot, but still, it's odd that a UK spot would have lasted longer than a minute (I wonder if they would have been able to squeeze the time to 2 minutes if they cut Kermit's introduction and/or the audience applauding at the end).