very cold weather “over base” shorts

I like the idea, gear re-use is not oft discussed. I would suspect in a year or two you'll improve the design and weight by using down and elastic cinch cords at the knee, shorts that double as your pillow at night.

During the last week of weather here in OK I have mixed my wool base layer pants,Houdini pants and a pair of nylon shorts (in that order) with great results.
Seems like it makes a "hot pocket" of air right around the upper thighs.
I want to try it this winter doing some actual strenuous activities to test this out.

I can leave the nylon pants at home and have a flexible system and save a few oz.
This set up in the chilly mornings then shorts in the afternoon.

There was a winter camping book from the 70's that had a pattern for down shorts. Keep the core warm like a vest for less weight.

Cloudveil made some heavy powerstretch boxer shorts for a while. They're awesome.

By cutting off the ankles, shins, and knees, they will be easier to put on.

I've considered trimming one pair of pants and one pair of exp-wt long UW to mid calf length for use with 12-inch-high boots. Mostly to remove bulk from inside the boots and to allow the pants to overlap the boots on the outside, but also to save some weight. ( I know, 12-inch boots aren't light, but 15-hour days of hiking on a grassy volcano in the North Pacific with lots of stream crossings precludes using my low-cuts.