Good memory, SassyLass.
I did do a long-ish flat tow, used a regular load binding/towing chain (maybe 3/8" link?), towed with a dual-rear pickup. Actually I've towed it a little with my single-rear Ram 2500 as well, but that was a little sketchy because of reduced drive wheel traction and because its cooling system (trans or engine, I'm not sure which) isn't what it used to be.
IIRC we did in excess of 40 MPH at times, but the brakes on the bus were fully functional, working on relatively straight and flat terrain with good visibility, and we literally did do some of the towing after midnight so there was minimal if any traffic.
I'd be uncomfortable about flat towing a bus without its own brakes at least mostly working. Based on my towing of 10-15k pound trailers with my pickup, it'll get the load going much faster than it'll stop the load because the truck's tires lock up and skid easily. My knowledge of 1940s era vehicles is zero but you mention the brakes are hydraulic. Do they have vacuum boost? If so you might use a portable vacuum pump and an air compressor tank, or the Harbor Freight 11 gallon air tank, as a way to build up a reserve of vacuum analogous to the periodic re-pressurizing I did with my air brake system. Either way I think it'd be time well spent to bring its brakes into functional even if not "road worthy" condition.
Once you get off road.... wow. Good luck. I tried to tug my bus (20k pounds) up a very very slight slope with a skid loader. It's slight enough that the bus won't roll down the slope without being pushed. The ground was fairly smooth, and the loader was working on a few inches of 2 inch minus gravel. It dug down through the gravel to the soil below and then just spun. It is 4WD like your NH, however its tires are only about 12x30 inches outside dimensions and the tread is shallow, maybe just 1/2". Your NH with its much larger tires may get much better traction than my skid loader did. I guess you could disconnect the drive shaft from the differential and get under there with a bar and socket to manually propel the bus!
The solid tow bar sounds nice so far as keeping the two vehicles from colliding. However, if the towing truck doesn't have enough rear end traction during a slow/stop, the bus pushing on a rigid tow bar might cause that rear end to break its traction and jack-knife.
If the hill isn't too steep, simple rolling friction might be enough to keep the speed manageable. You could try coasting the pickup truck down the hill on its own and see what speed it reaches.
With the front bumper potentially not being a robust tow point, what about the spring hangers for the front axle? You probably wouldn't want the chain(s) sweeping side to side under the front overhang of the bus though; maybe it could be secured near the front end of the frame rail to prevent lateral movement. Another piece of chain wound around the frame rail and towing chain, for example..?
As always, don't forget weights like a blanket, spare chain, etc draped over the towing chain/strap to dampen its recoil in case it snaps, particularly on the off-road climb where towing forces will be higher.
Taskswap's suggestion to take it 2 MPH sounds extreme, but if your patience can bear it, spending an hour actually doing the tow really wouldn't be so bad. At just 2 MPH it probably wouldn't coast far before coming to a stop on its own.