For what you want, an Integral Designs North Twin would be ideal and it is rated to 10*F, my experience is that ID bags are like Western Mountaineering and will work fine at lower than rated temps. The Andromeda Overbag for this, no zippers or anything is also a fine summer bag and the combo would weigh around 5 lbs. for a sub-zero bag that will perform in wet conditons.
I can honestly say, owning and using WM, FF, ID, Exped and having had a whack of others, that this is the most comfortable and most utilitarian bag(s) I have ever had and the quality is equal to WM. Something else to consider if Kifaru can't do a custom rig like you want...and I would as well.
