I have heard of a few orange tractors pulling orange disks very straight without aid of constant lever use. They were adjusted properly and the tractors close to being new. Only downside was the motors on the HD-16Bs rolled way too much "coal". Had to re-set several offset discs here, as the dealership did not follow the factory settings.
These tractors were also in large open fields that were previously laser leveled, so everything was perfectly flat.
There used to be some hydraulic assisted guidance systems for row crop tractors, downside was if the operator did not make the first row straight, it was worthless, as the unit followed whatever they began with.

The reliable old disc is the worst offender in giving side loads to a tractor.  One notices it most behind the old Cats as to turn them it is all or nothing.  Even a wheel tractor experiences the side load to the drawbar but on the wheel tractor it is easy to compensate for as one only has to adjust the steering wheel slightly to compensate for the side load. The tractor designers realized this was a problem so they connected the drawbar of the tractor way up underneath the tractor somewhat closer to the center of "pulling". Unfortunately there are so many forces at work such as terrain, compaction etc that the forces on the tractor are continuously changing. 
Back to your point Neil it would actually be possible to control the tractor via a hydraulic cylinder attached to the swinging drawbar.  In fact for small corrections one would only have to move a hydraulic control lever to move the drawbar angle.  The slight change in angle would change the drift of the tractor. Of course one has to remember the original steering levers would be used to turn corners etc. Back in the old D2 and D4 days we actually pinned the drawbar at an angle to minimize side draft.  The drawbar on the old Cats were not connected near the center of "pull" and we compensated for that with pinning the drawbar offset.  It was still pretty easy to turn corners as the disc comes out of the ground when turning as it is merely rolling and not cutting. Setting the drawbar angle was kind of like balancing a marble on a hill.

Hi, Neil.
That sorta system was 'ackshully' fitted to summa the 'ice tractors' in Antarctica and Greenland to help them turn with the heavy swings of sleds. If my memory serves me right, it is fitted to the LGP D8 up in Alaska.

I did a LOTTTA hours with both wheel tractors and crawlers pulling one-way disc plows in my younger days. I learned to 'set' a one-way disc plow so that a wheel tractor would run straight with the right front wheel running in the previous furrow and it would steer itself = sit and twiddle thumbs until next corner.

I also set up 2 X 14-disc one-way plows in tandem behind a 2T D4 so that the tractor would run pretty straight - did still hafta make small corrections 'cos MANY clutch-n-brake steering crawlers tend to 'wander' a little to the side to which they turn most after a while. I don't know if the Cletracs with differential steering did this or not, although I have heard that the Cletracs had a tendency to wander anyway.

Just my 0.02.

A host of things come into play as to how good it would work. A disc is probably one of the harder implement to keep straight. The operator would need to fully ungauged to start the correction at a early point to not have major overlapping or skips. Field cultivators and chisel plows I would think are easier. There are no flat square fields in my area, but as a teen my dad pulled a one way disc out of the weeds for summer fallowing one spring. Because of changing soil conditions it would cut 2 or 3 feet wider in mellow soil as compacted soil. Was a real snake trail to follow after not many passes. Never had the desire to try it again. As we had a heavy Towner 12 inch spaced disc already, just took a bit more tractor.

In orchard discing with the up and down hill as there is no contouring, cross steering is common by the more experienced. You pull the opposite clutch on a steep down hill and it speeds up rather than slowing down so that side gets ahead. In open field work I am big on contouring with a cultivator that you can turn ether direction and do a lot of back a forth patterns. Discing I stick mostly to the circle turning the offset disc to the left all day.

Another bit of steering without using the steering clutch is running the tilt cylinder on a dozer blade so equipped. A neighbor wanted a pit dug in a wet area to see if it would get standing water for wildlife watering. I had just added a joy stick to the D6 9u dozer and a tilt cylinder. Very easy to use the tilt to veer right or left coming out of the trench with a load of mud.

yes i think it's doable as well Neil, probably take some fine tuning and practice time....

but would definitely be worthwhile as it would eliminate a lot of clutch and brake work...

Hi Team,
back on the other BB we had a big session on Cat machines in the Antarctic for the International Geophysical Year or Operation Deep Freeze back in 1957-8 and a few of the Big D8's had a hydraulic cylinder connected to the swinging draw bar to steer the machines towing several sleds by off setting the centre line of the pull and so slowly turn the machines whilst traversing without steering clutch/brake input.
For ploughs etc the draw bar should be left to swing according to Mr Caterpillar.
An article appeared in a Dealer Magazine of the day--cannot find my copy at present and lost my scans of it in a 'puter crash-- on the swinging draw bar called something like "The Tail Swings The Dog"
Cheers,
Eddie B.

Hi, Neil.
4-furrow plow????? Is this a moldboard plow? They don't normally have a whole lotta side draught in my limited experience of them. Usually, it is discs that do funny things to steering and this can often be largely if not completely countered by doing a little 'fiddling' with the adjustments on the implement's drawbar.

In the tandem plow setup that I mentioned earlier, there was no draught beam and no chains or cables. The back plow was pinned to a fabricated drawbar rack on the back of the front plow and adjusted so that it ran its front furrow wheel in the last furrow of the front plow. I cut a few more holes in the draught beam of the front plow so that I could move that plow's drawbar across far enough to balance the load of the back plow.

That was 58 years ago and I can still remember it like it was yesterday. I was 18 years old. I was one happy little vegemite when it WORKED. The 2T D4 pulled it quite happily in 4th gear

Just my 0.02.