Miss Piggy appeared on July 19, 1999 to promote Muppets from Space.
Kermit the Frog appeared on February 13th, 2001 memorably to promote nothing in particular. He did say a new Muppet Show was happening, and low and behold 14 years later he was right!
and Brian Jay Jones appeared almost exactly 2 years after that to promote his book Jim Henson: The Biography. Unfortunately we couldn't find a clip from his appearance online.
There were may unofficial "Muppet" appearances as well:
After using a Whatnot from FAO's Muppet Workshop to mock then RNC chairman Michael Steele...
...the entire cast was turned into puppets via the talented people at Puppet Heap (who maintain The Muppets).
For a complete list of appearances, mentions and parodies check out the Muppet Wiki.
We wish Jon all the best!
Here it is, your moment of zen: