Talondale,
After reading your description of what you're looking for, I don't think you can beat some spurs and either a light hangon, one of those "hanging-sling-things" (forgot what it's called - linked above somewhere), or maybe a strap on seat. I use just a simple strap on seat with my spurs quite often. I either find a place I can strap on the seat with a branch below for my feet, or I put in 2 screw in steps for my feet. Super light, super portable. NOT the most comfortable setup.