So if I understand you correctly, you are happy with your SilPoncho and wish to continue using it, but would like more substantial options for underneath your poncho tarp setup. Is that right?
If so, have you seen this?
I am also experiencing some confusion with your comment: “a set up that utilizes part of the tent as a poncho like I can for the floor of the Zpack”.
Since Zpacks only sells the poncho ground sheet as a optional accessory, it really isn’t integral to the tent, and can be easily used with any floorless shelter.
Regardless, (as noted in posts above) I would imagine that many folks who own the Zpacks poncho groundsheet likely avoid using it as a dedicated ground sheet due to it’s expense and the increased risk of puncture. In general; ground sheets are really meant to mitigate abrasion as much as keeping water out. One hole or tear in the wrong spot could make it a useless poncho and groundsheet. Other cottage makers out there probably believe the same thing, and think it’s too much of a financial risk to develop it.