On the topic of sleeping bags - this is a clever idea.
A half insulated (bottom only) bag, since you are likely to have a down jacket in these conditions anyway, you save half the weight of the bag.

http://www.alpkit.com/shop/cart.php?target=product&product_id=16329&category_id=253

ps If you are in the UK (or Europe) alpkit do a small but well thought out range of gear, mostly for climbing/mountaineering that they design/make themselves. (Just a happy customer etc.)