I haven't tried this over boots, but I recently unstiched the sleeves from a nice old wool sweater and wear them armpit side up as leg warmers over leggings around the house. The shape of the armpit covers my knees. Really cozy!
I have to do this for my daughter. I'm thinking "what if you button these sleeves onto the arm holes of a vest, or different sweater"? What if you use ribbon and whip stitch them ballet style for leg warmers? the arm of an old sweater and sew on buttons. So stinkin cute! I wish i was more crafty