I would say that you want foot gear in any environment that will allow you to walk at least five miles comfortably, at a minimum. If I have to wear something that isn't,Itry and throw at least a pair ofsneakers in my bag or back seat.

I'm going to second the notion of rolling it instead of carrying it. Most jogging or all terrain strolles are rated for 40-45 lbs per kid and 10-20 lbs of cargo. My double stroller is rated for 45 lbs per kid and 20(?) lbs of cargo. It weighs 36 lbs. ATM, it weighs 90-100 lbs when loaded for a daytrip. I can cruise on flat ground with it. The only time I really feel the weight is going uphill, and my exertion level is considerable lower then carrying half that weight.

Conversly, carrying 50 lbs on concrete is going to kill your knees and hips.

In addition to a folding game cart, which is probably the most off road capable, there's a couple of other option:
1) Folding luggage cart: Drop the backpack on it and wheel it around. Strap it to the pack when the going gets rough.
2) Trolley cart: If little old ladies can push around a weeks worth of groceries in these, you should have no problems with an oversized BOB.
3) Garden cart: My neighbor tows his two grade schoolers around the neighborhood in one of these. Not compact, but can haul an obscene amount of weight.
4) Dedicated rolling luggage: Try to find something with inline skate or scooter wheels that can be replaced in the field and carry spares.

I have a light-duty folding hand cart that folds very small and flat, runs well over pavement and reasonably well on gravel, and is rated for 150lbs. In addition to regular hand-truck duties I combine it with a milk crate and some bungee cords to make my redneck three-gun cart. It's cheap, small and light enough to stash in your trunk if you needed to switch from driving to shank's mare.

there are a lot of options out there.

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.

A cart makes a lot of sense, especially in an urban environment that is for the most part still intact. Even in rugged terrain it could be helpful, but within limits. Maybe there'd be a way to rig up something you could pack on your back if you had to, and when the going is a bit easier you can put it down on integral wheels.

The Burley cart Dagny referenced to still interests me, especially the single point snap in attachment to the bicycle.

Suppose you could rig the part that attaches to the bicycle to an individual belt or harness? Then attach the cart bicycle style and you have a sort of travois or cart that you pull along behind you as you walk, leaving your hands free, but still could drop/disconnect quickly.

It should be possible to set up something like this with other type wheeled carts.

"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.

Something easier to doff than shoulder straps. I am thinking something like hooks that I could shrug and twist out of. Maybe I could rig up a harness with straps that had a hitch system with some sort of rip cord or button popper that would pop the hitch open and drop the cart off the harness clearly.

I'll have to think on this some more.

Your post made me think way back. This military gadget was a cam on the shoulder strap that could be quickly unlatched with either hand (it was placed on the front of the shoulder strap at about the nipple line. Once you popped that latch, that strap fell away and a simple shoulder shrug got rid of the other.

ai think it was issued sometime in the 1950s. I am not kidding when I call myself a geezer......

Very interesting looking, never heard about these. Found a shop here in Finland that sell that Travoy, price is quite high though, 360 euros.

I, having only my own feet to get around with, would have great use of that trolley/cart.

http://velosport.fi/verkkokauppa/burley_travoy-p-24596-14/