I have to pull the pipe to sweep my chimney so it's a none issue for me. For those that don't want to pull the pipe I think you can probably make a thin sheet metal deflector of sorts that you can fit up the bypass opening to direct the soot in front of the bypass plate so the soot can't fall behind it and get in front of the cat. It may take a two piece design but I'd think something can be fabricated.
Stuff a rag up there in the right spot too. The flue is quite rearward on the princess.