Forgot to add. The side MOLLE that replaces the ALICE webbing I mentioned. That may be an option for increasing its ability to work in cold weather. Slap the sustainment pouches off the MOLLE ruck on the side when needed and that can make more room in the main compartment for a good sleeping bag.
I’ve been thinking of basically the same thing. Unfortunately, the medium Alice can fit 2, maybe squeeze in 3 columns of MOLLE webbing in that space. The sustainment pouch is 4 columns. However, if you can squeeze in 3 columns, there are still a good number of decent size utility pouches that can go there. As a heads up, TT custom shop puts in 2 columns on medium alices when that mod is ordered.
On the plus side, there is a lot of space between the bottom of the frame and the bottom of the pack when mounting a medium Alice onto a DEI 1606 frame. This is space that can be used for a compression sack in the event of cold weather.