Yeah, you could go with just a bivy, no problem.
There are a few eVENT bivies made by integral designs at around 18oz, but I dont think they have bug netting, but do have the best breathability by far.
there are also 6-8oz bivies from montbell with no netting, but with a head net, they would be the lightest option.
REI has a minimalist bivy at 15oz with bug netting, looks good from what I saw at the store, but it may not be that breathable
There are also great bivies at owareusa.com at about 18-20oz which come in either eVENT or Gore tex waterproof tops
also available at REI, there are two 18oz bivies from mountainhardwear one goretex paclite, and the other Conduit SL™/nylon.
and the outdoor research basic bivy would be a good option at 20 oz
But if I were you I would go for the lightest montbell bivy, I have one, the wide and long is huge, even big enough, to store some gear, all you have to do is get a cheap headnet, and cover the face opening with your rain jacket, and you have a ultralight, breathable, simple shelter.