It's always seemed like a very good idea to me but there's not many out there. The only ones I can think of are the Crux Torpedo and the Ajungilak Shield. Neither of them are cheap but they would take the worry out of using a down bag in bad weather ... you also wouldn't require a seperate bivvy bag, so there might be a weight saving too.

Are there any others apart from the 2 above?

I've discovered another ... Mountain Hardwear SL series. Cheapest one's £430 though

Anymore?