Will you be carrying some sort of insulated clothing anyhow? (Something along the lines of a Patagonia Micropuff or similar.) If so, you may be able to get a lighter-weight sleeping bag by gettting one with a higher temperature rating (say, a 30 degree bag instead of a 15.) When temps drop below freezing, wear some or all of your clothing to bed, and wrap your feet loosely in a T-shirt, poncho, etc., to fill up the air space at the foot of the bag. You can probably add 5-10 degrees to your bag's rating this way; if you sleep in a tent or a bivy (maybe, with a windbreak) you may be able to add another 5 to 10 degrees. I currently use a 40-degree WM Mitylite (hoodless, semi-rectangular) bag, and have successfully slept in a bivy sack, with a set of R.5 longjohns, R1 fleece, and balaclava in 25-degree weather.
You should, if possible, try this at home before you leave, just to be sure it will work for you (body type - warm or cold sleeper - will affect your success.)