P0420 - Clean catalytic converter using citric acid spray, no removal

[leakyseals]

Bought a 2002 XLE 4cyl w/137k cheap because it was failing emissions with a P0420 Cat replace diagnosis from a mechanic. Was able to determine that the CE triggered on longer highway trips. Reset light, got my sticker. I know Toyota is honoring cat replacements up to 80k, but I have never replaced a cat, even at mileage close to 300k. Has to be a reason why the efficiency is going down.

Analysis of car: Second owner only put 30k around town miles in 3 years, rarely went on highway. Only maintenance was 5k OCI's, never checking oil or coolant levels, going half the OCI on dirty oil that was down a quart. Sludge buildup was pretty bad, PVC was stuck, MAF, plugs all sooted over, goes on and on.

Performed the usual tricks to de-sludge engine, stop blue smoke, unclogged PVC, cleaned TB, maf, plugs, etc. Runs great, CE light still triggers on the highway, but goes off on its own after leaving the highway, something it didn't do before. Progress.

Now that the reasons for clogging have been mostly resolved, and I know its efficiency not malfunction, I want to clean the cat rather than replace it. This is where I need advice. Rumor has it citric acid can degrease/clean the cat without damaging the core. Its the manifold cat, taking it out and giving it a bath is not so easy. Because the cat is aligned vertically, I was thinking about spraying a citric acid powder/water mix from a garden sprayer with a flexible nozzle down the upstream 02 hole just above the cat core. Let it drip through overnight, fire it up heating the cat, which produces steam further cleansing, eventually burning off.

My concern is how a wet partially clogged cat will behave when I start the engine...

1) initial back pressure that could fracture the core?
2) turn any ash/phosphor/carbon into a partially dissolved sludge +#1?

If anyone has any experience with this please respond, thanks in advance.