camping in fog?

[Cannibal]

Were your quilts down, synthetic or both?

Yes!
Warbonnet Torso UQ - JRB RMS TopQuilt

If it was down, and that had gone on a few days longer without the ability to dry out the insulation, would you have had a significant problem?

Probably, but hard to say without it actually happening. I think as long as the temps stayed close to the 30F - 40F range I probably would have been fine, but if a real cold spell would have rolled in I would have had to start taking additional steps like the old hot water bottle in the crotch.

But, unless a person is really on an adventure I think it would be difficult to not find a way to dry the quilts. I took advantage of every ray of sunshine those days. Really pointed out the advantages of having black material utilized in the quilt (half of it at least); soaks up that sunlight as quick as the down soaks up the wet.

[Youngblood]

In your experience with the above, how does that work out with your down insulation?

That's a great question. When that happened on my 2000 AT thru hike it soaked my sleeping bag. It even penetrated a zip lock bag that had too much wear on it and soaked my cell phone and camera to the extent it damaged both while I was in my small tent on a miserable day/night (or 2?) of light misty rain with the air saturated with moisture. (I now use a small wet bag type closure for my camera when it is damp and put my cell phone in a zip lock bag that stays in a silnylon stuff sack... hopefully double protection. But change out zip lock bags before they wear out on stuff you really need protection for.)

When I finished my thru hike I replaced my sleeping bags with lighter, smaller girth, more expensive bags. The newer bags have much improved DWR and I haven't had them soak through. The outer surface gets damp but you can shake most of that off, it doesn't seem to soak through unless you apply some pressure to it. My body heat keeps the inside insulation seeming dry, at least it lofts and keeps me warm. Those were Western Mountaineering bags with the very light weight fabric. The DWR on the current rip stop nylon quilts and under quilts I have seen seems to be comparable and I have been through similar conditions with prototype SnugFit Underquilts where the DWR on their RSN seemed to keep the insulation from soaking through.

The DWR on the RSN is not water proof and while it will shed water in a lot of cases, it will only do so much. It can wet through and it is a matter of how much, how long, and how much pressure. While I haven't had the current type of RSN with DWR soak through, that doesn't mean that it can't and it might in other situations that are a little different. Fog or high humidity or misting are different from ground splatter or dripping water and it wouldn't surprise me for the DWR on underquilts to be okay for the former but not so much for the later.

Just for interest. I used a Patagonia Zepher jacket that I bought circa 1999 on my thru hike and still use it today. I bought one for a girl friend that I backpacked with circa 2001. I recall us getting caught in light rain where the DWR on her jacket repelled water while mine just soaked through very quickly. I think sometime around 2000 they made significant improvements in DWR finishes.