I've been mulling over a similar idea, Tim. Except my weight expectation is not as ambitious, and I'm setting a pretty low budget.
To answer your question, I think 1200 grams is possible, but only at high cost and not very strong. The way materials technology progresses, I'm sure that will change by 10 or 20 years from now. I bet we will consider 1000 gram wheelsets to be an everyday, mid-budget option.
I have a generic front road hub that's probably less than 200 grams, including skewer. I thought I'll get a Gran Compe SF rear because they're light, cheap, and good looking. Also, because matching (they look almost like my front one).
I've looked around at lots of rims. Pacenti Brevet seems to check all my boxes. They're not aero, but they're about 360 grams, about $60, they look super nice, and TUBELESS. I'll have 28 spokes on the front and 32 rear. The set should come in at no more than 1700 grams and cost less than $300, built by my own bad self.
I believe I can save a little weight by going tubeless. Tubeless version of any tire is a bit heavier than the tube version. You save weight by leaving out the tube, but that's partly negated when you add sealant. On the whole, a tubeless setup is probably very similar in weight to standard clinchers with 100 gram race tubes. But here's the caveat: I ride heavy-ish puncture-resistant tires (Gatorskins currently) and I also use heavy duty tubes--heavy being the key word here, because I pick up lots of thorns where I often ride. Compared to a puncture resistant tubed setup, I think I can shave at least 100 grams off the complete wheelset--probably more than that. Better ride quality will be an added bonus, but that's a completely separate topic.
This is what I plan to build later this summer. I think these wheels will be an ideal balance of low price, low weight, high strength and durability, and good looks... plus punctures will fix themselves! I'll probably post the finished products once they're done.