I LIKE the idea of soaking or dipping them in a solvent that makes the rubber get soft. That is a great idea. I am not a chemist though and I don't know what would work. I'm gonna buy a repop trunk gasket, cut it in 2" pieces and test them against time, soaking in mineral spirits, a/c compressor oil, lacquer thinner, MEK, toluene, and xylene.
One of them is bound to get the desired results. I'm concerned that the absorbed solvent will ruin new paint though. I guess belt-sanding off some of the rubber on the mounting face would be the 100% certain solution. It would just take a lot more time.