Interesting idea, Seven - Although the combination was a tight one, the wicking action of liquids in cloth obviously allowed the patch to dry out - I saw it and it was dry- with rust. The combination Taylor listed above would be difficult to seat in a bore without lubrication. .495" ball + .023" patch = .541" in a .524" groove diameter for .0185" compression per side. That is a load that seals & needs lubrication to seat. Since we don't shoot combinations that would allow loading in a dry bore, we won't be testing a dry bore. In this case, the powder's chamber was sealed between the breech block and the patched ball. It did not rust as no condensation reached it - apparently - or did that area dry through the patch? That the patch must have absorbed moisture and dried slowly, causing rust seems evident. That the bore itself was protected somewhat by the 'dry ed' LHV then, exposed to air and thus dried quickly & didn't rust is possible. But- what about the powder chamber where the moisture present could not have evaporated, except that the moisture may have gone into the patch as well, seems correct, where it then sat against the bore, causing rust.
The first test of 5 days showed no rust. We must remember that. That barrel was left out in the 'elements' for 5 days, although the temperatures were not extreme - just barely freezing to about 40F during the day = little or no condensation as the temperature changes were gradual - the kicker?. Were that 5 day experiment redone in temperatures that run more normal for winter here, such as -20 or colder at night, then brought back inside the house producing much more moisture through condensation than the second test provided, we may have witnessed rusting in that short test. Don't know.
To repeat the tests with grease of some sort - mink oil, or some concoction of beeswax or SPG and neetsfoot oil - something like that. I've left the .69 sitting for many months, loaded with a mink-oil (paste until touched by hands) lubed patch with no rust. It sat in the house after being out hunting for day, from October through December and on into January before being discharged - no problems with accuracy and no rust occurred.