I mentioned in my post that Luckovich was the organizer of the tribute and Patrick McDonnell suggested the idea. This is according to a letter written by Luckovich that appears in the Tribute to Sparky book.
(Not all of the tributes are reprinted in the book- only those that the original artwork is in the collection of the Schulz Museum. A number are autographed by the cartoonist.)
On a sorta-off-topic thing, also interesting to note are tributes sent by three animation studios- two were get-well cards and one was a goodbye message when Schulz announced his retirement. Warner Bros. sent an original drawing and message from Bugs Bunny, who is apparently saying the message into a doctor’s stephoscope (the doctor is noticiably jarred): “Testing one…two…three…eh, what’s up doc? (Couldn’t resist) Be sure an’ take good care of me ol’ pal Sparky.” Walt Disney Studios sent various personal messages from animators written around an original drawing of Mickey Mouse doing the Snoopy dance. When Schulz retired, the animators of The Simpsons did Schulz one better, sending a card filled with personal messages and sketches. (Homer with a Charlie Brown head, Woodstock and Maggie speaking in the Woodstock “!!!” language, etc.)