I think this line of thought is smart and can prove quite useful.

Back in the day, both Kelty and North Face made waist/fanny/lumbar packs that converted into daypacks. A zipped compartment contained the upper pack body and some flat shoulder straps. It was pretty cool but I don't think they're around anymore. In recent years Maxpedition made something similar called the Lochsa pack. It was bulky and heavy, like most Maxped products, and I think they discontinued it.

As long as such a pack's weight/bulk fit your profile it could be a very versatile setup. It would be awesome to have one sewn up with modern materials.

I found some photos of the Kelty item on a Japanese website: