I've been looking into this as well, less for weight and more to get the most compact tent possible. As far as I can tell, the smallest packing double-walled tent is the Big Agnes Seedhouse SL1.
A few things about bivys. They're single-walled and very tight; most bivys barely wrap around a sleeping bag. Changing clothes inside one will be quite a challenge.
Condensation is also a big problem, especially during a heavy storm when you'll want to seal up. A tarp-bivy combo is good, but that cuts into your weight savings and takes longer to set up.
Third, in general ultralight gear is not going to be very rugged. You'd have to repair or replace an ultralight shelter long before something more standard (i.e. heavier). You're paying for the weight savings not just with the price tag, but with the robustness of the gear as well.
The most promising mini-tent/bivy item I saw was the
Nemo Gogo, which uses a pneumatic tube instead of a tent pole. Packs down very small and light, and is roomier than things like the Black Diamond Lightsabre Bivy (but not as much as some other hoop tents). I haven't used it, but some reviews point out that it's not particularly tough, and condensation is a challenge (as with any single-walled bivy).
