Why worry about getting a little water under the ribs? You're likely to get water in there anyway when you neutralize and rinse your after browning or bluing. Simply heating them to near boiling temperature will quickly turn any water to steam and drive it out. Just don't get them hot enough to melt solder, which typically would be over 360 degrees F. The rib between the barrel and ramrod on my flintlock rifles is screwed on, so rain or water from cleaning can easily get into there, so I always set my cleaned barrels next to the furnace plenum to dry thoroughly before oiling them.
Quite a few barrels have at least one or two small gaps or pinholes in the rib solder joints. The bigger challenge is getting all oil and contaminants out of there prior to doing your browning or bluing rusting cycles so it doesn't creep out and screw things up during the process. This is why many barrels that have been reblued have a small weep hole drilled into the bottom rib.
