Aha!

I remember this episode!

Against Big Bird, The Gods Themselves Contend In Vain

There are too many Sesame Street specials to count, and most of them have climactic revelations along the lines of, "Oscar learns that not everyone likes a grouch" or "Elmo learns the real value of sharing." Yet once, just once, a bunch of writers at the Children's Television Workshop actually decided to run with "Big Bird overpowers the will of gods and demons in a quest for celestial justice."

Okay, so I didn't imagine that story! Was there also a bit either in that episode or elsewhere, where Bert and Ernie run into a mummy? Possibly an invisible one? Or am I really imagining that part?

ETA: Aha, scott_lynch comes through again:
"I remember the skit very well; it was from a regular episode:" http://www.youtube.com/watch?v=8yiqGtZXCmQ

Yes! And as I mentioned in the thread, I think it was those two episodes I was conflating. Because I think I saw the mummy one later or was talking about it with someone and I kept thinking that there was a part with a little Egyptian ghost boy that I was missing...