Per Michael Daly:
Your pants don't have a zipper or flap?

Not so far. The couple pair of fleece pants I have don't have either.

Next time I go shopping, though, that'll be on the "Must" list.

Personally, I greatly prefer a one-piece liner. They're available in different
weights from summer undergarments to winter fleece.

Got any brands in mind? Do you reacal what the term was to refer to them?
"Union Suit"? "Farmer John"? "Coverall"?

How about putting some
buttons or Velcro on the pants and top to hold them together? If you add
Velcro, put the hook side facing out.

My sense is that wouldn't work so well given the stretchability of the shirts
I'm wearing now.

Sounds to me like 3 coveralls with a fly opening like men's briefs would
probably do the trick: thin, medium, and heavier.
--
PeteCresswell