"Assume you have a tarp to use with it."
OK, now I get it.
I like the idea or some variation on it. I don't like the idea of using my tarp for a raincoat because the raincoat gets a lot of abuse and I don't want to end up at camp with a torn shelter. I'm much more careful with my night shelter from the rain (tarp or fly) than I am with my daytime rain shelter (raincoat).
But your idea only exposes the non-waterproof(bivy) portion of the night shelter to the rigors of day use and it is carried inside the raincoat so it is well protected. My concern in the paragraph above doesn't apply.
One might also be able to morph the raincoat into the floor of the bivy, thereby making best use of its waterproofness.
Good food for thought. Thanks for the post. It's a subtle change to an old idea that has promise, in my opinion.
Also I'm not concerned about getting too warm with rain gear at all. I put rain gear on to stay or get warm not to stay or get dry. I'm always wet from either rain or sweat.