Just looking for information on teh subject, I have a bunch of apple and plum trees which are hollow (>> 100 years old, I live in Scotland, the house was built sometime around 1700) ) and they seem to be doing fine. There was one tree which was particularly bad. What I did was put a wire brush on a drill and got rid of all the soft moisture retaining wood. Then vacuumed our all the dust. Essentially just keep it clean. The good wood that's left seems to go as hard as steel and the rot stops. Other than that I just keep them pruned so the wind is not to hard on them. And most importantly learn to love the imperfections.