.....I've just bought one of these too (Ajungilak Grevling)...
http://www.mammut.ch/ajungilak/katalog.asp?view=detail&did=36&dart=3&tid=1875
Which is supposed to add 2-3degC to the bag. Should be toasty. At 86cm x 220cm, it should fit over the Osprey (82cm x 210cm approx) nicely, with a bit to spare. BTW, has anyone tried the Grevling? I bought it as a sort of "fair weather bivvy cum bag protector" for lowland, 3 season use. The base is waterproof, but the top is a water-resistant, breathable poly-cotton. My rationalle is I also have a waterproof army poncho, which I could throw over the top in a real and unexpected downpoor, but for normal use, this lightweight poly-cotton bivvy, plus hootch would be enough. It might even have an element of spark resistance for bivvying near an open fire. It was £40.