I think it's an interesting idea. The Zpacks poncho groundsheet seems like a fairly well regarded multi-use item as is, and my bivy sack more often than not just gets used as a groundsheet anyway, so coming up with a poncho/bivy could be a neat piece of gear.
The main issue that I could see coming up is how best to deal with the cuben/netting top when using the poncho/bivy in poncho mode. You'd essentially be wearing a double-layered cuben poncho... Would that affect the poncho's ability to vent?
It would add weight, but something like a full length waterproof zipper down the top of the bivy and some loops and toggles along the floor of the bivy would allow you to roll back and secure the two halves of the bivy top to the floor (like a tent door) for use during poncho mode. This would come closer to allowing the poncho/bivy to function like the original poncho/groundsheet.
Anyway, I'll be curious to see what you come up with and how it works in the field.