Well, a couple of comments. Tompions are not normally used for long term storage of a firearm out in the weather. They were issued to the military, and many high end, cased, English guns came with tompions to keep dirt and moisture out of the bore while in the field. Second, unless you have had direct experience with wood (walnut, oak, or whatever), actually collecting moisture and causing rust, I would differ with your opinions. If that were true, gun barrels would rust out from the outside in and on the bottom due to contact with the wood stock. Most original guns I have disassembled, unless they were seriously abused and just left in a terribly wet environment, show exactly the opposite. The barrel steel is in the best condition where it has been in undisturbed contact with and protected by wood.
To each his own, but I have never seen any degradation of the barrel in contact with a dry tompion. If, of course, you leave the barrel wet for long periods, with or without a tompion, you will have a rust problem in the bore.
I doubt the very best English gun makers would have supplied tompions with their best firearms if they were a quick way to rust a bore.