All the scanarios I tried using a bug screen incorpotated with a ponch would be as heavy as just using a light bivy with a poncho. The bivy/poncho combo is less clumsy and more versatile I think. On fair nights, the bivy could be used alone, making for a quick and easy setup.
I've run through a bunch of scenarios using the Gatewood Cape and various insect netting schemes. The lightest is a head net and progressing to partial nets on up to the Serenity insert, which gives some ground protection in the bargain and 8oz weight.
Borrowing from the hammock world, Papa Smurf turns out a sock style mosquito net using tulle netting that are 3-4 ounces. Tulle will stop mosquitos, but not noseeums. It is generally a big tube with a drawstring at one end.