Rav Yisrael Greenberger, rav of the Vizshnitz communities of Petach Tikva and Elad, has paskened that brushing one's teeth on Erev Pesach to remove any chametz is not enough preparation of one's mouth for the holiday but one must brush his/her teeth to remove any bits of chametz stuck there and then fill the mouth, as much as each person is possible, with boiling hot water and then spit it out.

In addition, according to Rav Greenberger, 24 hours prior to the time listed as the final time for eating chametz one is prohibited from eating any hot chametz, the problem being the hot food leavign traces absorbed into your teeth and the difficulty it causes in removing it. You would not be allowed to eat hot soup with bread or croutons, for example, or a hot pastrami sandwich, or hot pasta such as macaroni and cheese. You could eat the chametz cold, separately, and the hot soup separately, but the hot chametz can't be eaten within 24 hours of the final time. If you are eating a hot pastrami on rye, or a hot dog, separate the meat from the break - eat the hot pastrami separately and the rye bread separately, though I am not sure where you would put the mustard in this example. No dunking your donut in your coffee on Thursday.
source: Kikar

I admit, when I first saw this article I thought I must be reading a comedy or humor page or maybe it is a late Purim edition, or that I landed on a spoofed satire site. Interestingly it actually seems real.

it does remind me of the old discussions regarding braces and if they need to be kashered for pesach or not. But Rabbi Greenberger is talking about everybody and anybody's mouth, even with no braces, even with no silver fillings.

His premise that the teeth absorb, need to be kashered, and that it is a problem, seems new to me. Why do we not have this problem daily with milk and meat? If you eat a hot meat meal, you would not be allowed to eat a dairy meal for 24 hours, not the 3 or 5/6 hours we actually wait.

Why does the shulchan aruch not talk about this problem - whether for meat and dairy on a daily basis or for Pesach? It seems it would be be a far more serious problem that the kashrus of pots and spoons of the wrong gender falling into a pot of the other gender soup.

Since when is the mouth, or any body part, considered a vessel that absorbs and has to be kashered?

I hope Rabbi Greenberger plans on taking responsibility for the people who listen to him who get injured with second degree burns, or worse. I hope he was talking pilpul and rejected the suggestion in his conclusion and made it clear to everyone that this should not be done.

If not and if this is this year's crazy chumra, next years will have to be an extension of this - you cannot touch hot chametz within 24 hours of the final time as your hands might absorb the chametz. Everyone must toivel in a boiling hot mikva before pesach to do hagala on their hands and other body parts that might have come into contact with hot chametz. Not just the mouth, but the entire cavity of the body, especially the tongue and the entire path down - the esophagus, the stomach, the intestines and the rectum must all be kashered for Pesach with boiling hot water. And coming soon will be telling everyone to also get a colonoscopy before Pesach.