This does not answer your direct question in the sense that it does not prevent condensation (so please bear with me), but..
..a lightweight bivy with a water resistant and very breathable (rather than waterproof) top fabric on top of a groundsheet will protect your sleeping bag from the outside and add a bit of warmth. These weigh from 100-200g upwards, depending on fabric thickness.
I got mine from Mountain Laurel Designs. Right now there is only one model, although he used to do a plethora of options - perhaps worth asking if you think it looks too flimsy or too expensive.
For example this one:
http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=22&products_id=30
Not cheap though - there is an older thread on Backpacking Light with a discussion and a few more suggestions: