I remember from the final days of the original Bloom County there were some strips of the characters looking for work in other strips. Oliver Wendell Jones Jr was depicted as Bill Keane would draw him for Family Circus and Steve Dallas was up for a gig as a boyfriend for Cathy. I don’t remember if there were any more.
On the occasion of Charles Schultz’s death, almost every strip in the paper did a salute to the dead cartoonist with visits to his grave site or pats on Charlie’s shoulder and the like. I was impressed.
My personal favorite was from when Steve Canyon and Lil’ Abner were still running. Lil’ Abner loses his job as cartoon character and goes to work in the Steve Canyon strip… The best part was the whole thing was drawn in the style of Steve Canyon.
I don’t know if it counts as crossover or parody, but Francesco Marciuliano, writer of the Sally Forth comic strip, publishes a blog where he sometimes posts wicked tributes to Charles Schulz’ Peanuts.