Felted sweater would work great for a stuffable-pad insert type thing! You'd want to top it or pair it up with a few layers of cotton, though, as wool is a slow absorber, and cotton is fast. You don't want any fluids accidentally running off, because there's nothing to catch a gush immediately, yk? But a layer of felted wool knit under a cotton insert would help absorb more fluid if needed, and provide more of a protective barrier for your clothes, too. Just make sure you felt it down real well - I made a diaper-doubler size insert for a WIO diaper once, and *thought* that I had felted it well......but it shrunk down to the size of a menstrual pad in the next 2 washes :LOL
For mine, I was just thinking of a backing of wool jersey or flannel, well felted. A friend of mine just sent me some tester pads made with a cashmere interlock for the snap-in holder, and the cotton pads were backed with a layer of wool flannel for extra absorbency. She said to wash them on cool, though, or they'd curl up, so maybe I'll just stick with the wool holders I have, and make more snap-ons.
I never did the frozen pad thing, or the herbal compress thing....but then, I didn't tear with my last 4 births, either, and was up & about before I should have been, because I felt so good. So some people probably really like the cold pads, but I could've cared less!