Down sleeping bag moisture absorbing cover?

On a recent trip I cowboy camped, clear night, 25 F.

The outer layer of my bivy had a lot of frost on the outside, also on the inside of the bivy.  My down sleeping bag remained dry.

So, a simple layer of DWR nylon functioned like you’re looking for.

That was at 25 F clear night which probably isn’t as cold as you’re thinking about though.

You haven’t given the weight of that T’Rest topper so it’s hard to give an answer and it depends on how much of a temperature boost you also need. what your budget is and how much bulk you are prepared to put up with.
Visiting the Cascades Designs website I can’t find such a product so I have to assume it’s a deleted line
If it’s similar to the Stellar blanket you are only going to save about 100 grams going to UL fabrics and APEX fill

For years, I’ve been using a S2S silk mummy liner wrapped around the outside of my quilt, for the exact purpose you mention.

Just got back from a trip and I’m always amazed how well it works at wicking moisture. The small parts where my quilt became exposed during the night were slightly damp on the surface, but both the silk and everything covered the silk felt completely dry.

And a liner can dry very quickly.