Have you thought about silk? Sierra trading post has Terramar silk base layers cheaep and they weigh approximately 3.5oz for the large top and 3oz for the medium bottom. I got them to wear and pjs in my quilt. I've worn the bottoms w/shorts and the tops under a Houdini + UL Down Inner parka down to around 40.
One issue is that the stretch in the cuffs stretches out very quickly to the point of being non-elastic, hanging sort of baggy around my wrists. Not a big deal worn as sleepwear or undersomething, but might get annoying if I worn it as my only outer layer on my arms. If I had the gumption, I could probably hand stitch them a little narrower and still have enough stretch to get them on, it wouldn't look nice, but they're base layers, right?