"That is an interesting scenario, but I really don't care that much about it. What I care about is if it gets damp, not soaking wet. For example, if I'm out on a week long trip, and it is rainy and foggy every day (where I'm essentially in a cloud), how well does the bag work? "
Essentially for that I take a bag with a little bit extra down as buffer. The los that I get from damp conditions effecting the down, is so largely or even completely buffered by that extra down.
OK, so I always take a bag that is a bit warmer than I need for most conditions, and is thus also a bit heavier, but I'm always comfortably warm and isn't that valuable too (to say the least).