The BF won't fold small enough. You can get almost any bike down to 8Kg if you are willing to spend the money to reduce the weight. Dahon's Mu Sl comes equipped about that light and would fit you. (Not great for heavy use, but perfect for what you want to do.) No bike will be very comfy on your back. Someone just posted about a $20 bike backpack.

You can also get a bike that rolls when folded.

Someone on the Birdy group (http://sports.groups.yahoo.com/group/birdybike/) is selling an 8Kg Birdy used. Birdy makes a much more expensive one as well that would better fit a smaller bike (and your back).