I have a huge maple that I improperly cut off a very large limb, didn't get the crown distance / angle correct, and rot set in. That was about 7 years ago, and a cavity developed before it could scab over. The decay seems to have been arrested by the appearance of snails(!) over the years. Their mucus appears to have stopped further decay, but the cavity does hold water, and the limb crown is not able to close up, apparently. Sure seems that it would be beneficial to close up the cavity by filling with something. How about expanding foam?