I like that idea. My intention was to make the ultimate cheap poncho liner/underblanket with the idea that it would be multifunctional as-well as being modular in each role. If in the underblanket role, someone found it a bit too cold, then they could add another layer of pile or other insulation in if they wanted to, or if out and about, it could be spare clothes.
I like your idea of cutting a v out, but then you wouldn't have a poncho and poncho liner, it would be a singular use item. I love my poncho around camp, especially with a liner of some kind. but hey you have given me an idea for another project!