I don't recall Fran Brill being asked about projects she didn't perform in. It turns out that she doesn't remember performing the drummer, and many sites in the past have listed her as the performer (the only time I can recall seeing anything that suggested she was performed by anybody else was in a Jim Henson Hour website that listed Rob Mills as the drummer, but I figured this was a mistake, though that was unsourced as well). Fran Brill either didn't remember performing her regularly, or she didn't have a regular performer, or she had a regular performer but Fran Brill performed her in the behind-the-scenes footage.
As for Kevin Clash, I did ask about most of those. I suggested asking if he'd performed in A Muppet Family Christmas. I also suggested asking if he performed in the 30th anniversary special, since Clash has often quoted being in charge of the performers in the finale (I just wondered if he actualy performed in the special, and I don't think he actually confirmed that he did perform in that interview, though I guess it's common sense to assume he did perform, as he'd had plenty of performing experience by then, I thuink he was a main Muppet performer as soon as he'd been officially hired by Henson, and the special would have needed as many extra hands as possible). As for Folow That Bird, Kevin Clash was listed as an additional performer, but I didn't know how little he performed in it (I was suprised that he voiced a board bird, since he wasn't credited under the list of Board of Birds). Steve asked on his own if he performed in Mupet Vision 3D, but I did feel bad about suggesting all those questions after it turned out that he didn't do much work in those (outside the 30th anniversary, of course). I had thought about suggesting a question about whether Fran Bril performed in The Muppets: A Celebration of 30 Years or A Muppet Family Christmas, but chose not to, just because of what hapened in the Kevin Clash interview (and no, if The MuppetCast ever includes an interview with Martin Robinson I won't sugest asking if he performed in those productions, either).