In reply to windjammer:
Other option, have a synthetic over bag.
You can get light DWR bivy sacks (MLD's lightest is ~120g), too.
A bivy bag or nice DWR shell will deal well with external moisture, but you've still got internal moisture to deal with. That's where a synthetic over bag comes in handy, and as a bonus it may have a highly water resistant shell and minimal stitching (no baffles) so provide some protection from external moisture too.
As to choice of overbag...I thought a synthetic quilt was a good option. Construction is simpler so they end up much cheaper, they're lighter and can be used alone in warm climates. In fact they are so simple they're feasible to make at home...