First, they did, indeed, riff on "This Island Earth" at ConventioCon II. I was there and it was hilarious.
Second, you forgot to mention this: as part of the abbreviated Season 7, MST3K ran the movie "The Incredible Melting Man". And they used the host segments to mercilessly parody the making of MST3K-TM, especially the interference by the water-guzzling suits at Grammercy.
@gmp2I was at the second Con in '96, too! Rather than doing a live performance, they showed MST3K: The Movie, as well as the deleted scenes. I've finally gotten around to scanning my Operations Manual from the con: http://sherilynconnelly.tumblr.com/post/59028727187/1996-mystery-science-theater-3000-coventiocon The live-riffing of "This Island Earth" was part of the first Con, in September of '94.
You're right; I stand corrected. I got my ConventioCons mixed up.
I do remember Forrester's entrance in the gold cape, Frank and the popcorn, and Frank's interpretive dance. And I still have the "MST3K has Universal appeal" button they gave us before the show.
@gmp2 I'm jealous! I wish I could have attended the first Con. I have to admit, I was disappointed when I heard they weren't going to be doing a live show for the second Con; I totally understand their reasoning, what with Trace leaving, and I still went and had the time of my life, but...