Pack Rabbit offers two different sizes of rigid pack frames that pull double duty as chairs. The larger size Pack Mule is used by Marines to carry breaching tools and odd sized loads. Sure, you can sit on it if you want, but the beauty is that it keeps the pack and frame upright when deployed.
Although it can be used as a stand alone frame, Sand Piper of California has developed a pack bag that integrates with the Pack Mule. The Pack Mule has over 20 attachment points to integrate a variety of pack bags or loads. The frame integrates some flexion and you can set the amount of tension in the lumbar in order to maintain contact with the back.
Available in Stealth Black or Desert Sand, it features integrated closed cell foam padding.