What!?! Rainy and cold at Mount Rainier in early summer? I am shocked, shocked to hear such a thing!
No, you re right; it s not surprising at all.
So, the Marmot Couloir. It s a serious zero-degree, 800-fill down bag ($400) with a water-repellent coating, so I shouldn t think that a little moisture will cause it much difficulty. If I hear you correctly, the outside of the bag is what got damp. If so, what is happening is that there s a little condensation forming on the interior of the tent, and some of that is transferring to the surface of the bag. The simplest fix would be to carry a small plastic garbage sack, and just slip that over the bag. That might in turn cause a little condensation from moisture given off by your feet, but not much.
If the bag is damp in the inside, then just put the garbage sack inside the bag and stick your feet inside. That will create a sort of mini vapor barrier liner. Your bag will stay drier, and your feet will be warmer, too. But carry spare socks as your socks will be damp in the morning.
Sometimes, though, damp just happens.