I wonder how much that any leftover flux from brazing might be hygroscopic, thus attracting moisture from relatively dry air.

Perhaps this would need to be washed out before any coatings could be applied effectively.

I've worked on many bikes where a pile of loose rust had fallen down into the bb shell, even raced one of them for years, but have yet to see a frame failed from rust-out.
I should mention that I live in California though, where humidity is very low most of the time.
Bikes exposed to garage or outdoor freezing temperatures might be much more likely to suffer from condensation in the tubes.