mev Do you ever use the SOL emergency bivys with the tube tent by chance?
I tried one of the non-breathable SOL bivy's a couple summers ago when I was at my cabins as an additional layer. That was a mistake - the condensation left my sleeping bag uncomfortably damp, so much so that I ditched that night and crawled in bed in the early hours of the AM. It seems to me I tried one of the breathable bivy's also, it worked better but still had some condensation from what I recall - not to the extent the non-breathable one did.
In any event, I am thinking if there was any wind-blown rain one of those bivy's might work as some extra water protection - but I suppose if theres dampness from the inside out from condensation there may not be much benefit.