Rain Poncho as poncho/groundsheet/beak for cat tarp?

Hi all,

I've been thinking up ideas to make my rain gear more utilitarian and multi-use, and I thought of trying to pick up a poncho that I could also use as a groudsheet (ditching my ~1oz. GG PolyCro) under my bivy or, also, as a beak to pitch to my cat tarp for hard rains.
Ultimately, I'd like to give up my rain jacket/pants for a poncho and rain wrap which would shed a whole handful of ounces and give my raingear more "jobs" to do instead of just sitting in my pack when it's not raining.
Is anybody doing something similar to this?
Right now, I'm looking at the AGG Poncho Villa and the ID SilNylon Cape.
Thanks for any advice you could offer.

Adam