Ive been trying to wrap my mind around moisture condensing in your underquilt etc . I know down especially will gain moisture and weight on an extended winter trip . It is not so cut and dried as one inch into your 5 inch quilt you encounter the dew point and bam your warm moist air will be forced to condense . ( I know you used the degree per inch to make it understandable ) . My experiance is with insulated walls etc . This is why Tyvek was developed to allow water vapor to escape while holding heat in and wind out .
I wonder if water vapor might travel thru certain materials and escape to the environement outside the quilt without necessarily condenseing .

Another point to remember as your warm moist air travels to the outside of the quilt it will encounter cold outside air that has traveled into the quilt . That air will expand as it warms up and become much lower RH as it expands .
It might be that water vapor would possibley mix with the formerly cold outside air at that point rather then condense on your quilt .

I plan on testing some of these theories in my extremely breathable but insulated sock . My thought is I can build a sock that will let most of the moisture out while retaining a lot of the heat so that frost will not form on the inside of the sock .