"I have in mind something I can put up on my shoulders or around my waist. Not necessarily to strap it on; something I can doff at will, but that will stay secure, like shoulder hooks or something. In transit, I want my hands free to be able to other things, or carry other things, whatever. But I don't want to be unable to drop the cart and run or dodge if I have to without undo manipulation."

Isn't that called a back pack, at least the fairly small ones? I can get out of mine pretty quick. From my army surplus days, I remember some (the old classic plywood pack boards) with straps that could be quickly disengaged, so the load could be dropped right away.