Yeah, at some point every person associated with MST3K has been suggested as a guest riffer. I think at one point they basically said that the door is open to these people but for whatever personal reasons they have yet to join Mike for a Rifftrax. Everybody here hopes the day will come that more MST3K alumni appear as guest riffers.
That said, if I were to choose a non-MST3K lady to be a guest riffer, I think Amy Poehler would be really good at it.