I would say, the fruiting habit is in the fruit, not the rootstock.
How would you go about taking rootstock?

Rootstocks are selected partly because they might be good 'pitchers' meaning they take root from cuttings. If you use rooting hormone and base heat you might be able to get cuttings to take root. Once you have one growing, you can coppice and mound soil up around the new shoots to get it to root farther up, then cut these off.

Most apple cuttings are very hard to root. Hopefully you have some suckers to dig up.

Thanks for the feed-back and questions.

Regarding the questions concerning how I plan to get some rooted cuttings off of this tree:

I have cut scion sticks off the tree in February of last year as well as this year. I have grafted these onto apple seedlings of a few years old. My plan is to let the grafts grow out for a year or two in order to get some length of growth to work with. Next I will lay the grafted sapling on its side and take an empty pot. I feed a branch or leader up through a drain hole in the empty pot and then fill the pot with potting soil around this un-rooted branch. If you keep the soil moist, the branch will root within a year. The next spring, I cut the branch just outside the drain hole and the newly rooted branch continues to grow. A new tree! :)

I have had a lot of success with this method.

Why do you assume that it is the roots that are responsible for the growth habit of this tree? On grafted trees the scion has at least as much influence on the growth habit as the root including it's relative vigor.

For instance, a Northern Spy on M26 will grow as vigorously as a Goldrush on 111. On any rootstock, N. Spy will produce extremely upright growth while Goldrush will not even require any branch spreading.

Some varieties of apples spur up and bare fruit when very young and precocity is dwarfing.

I like the industriousness of your rootstock production technique and I hope it is the root that is the source of the desired traits you describe. Experimenting is the only way to find out.

Most likely, a roadside tree is a seedling, but no guarantee, no guarantee it will root either.

It doesn't mean this apple will root, many don't, even with your technique, this is basically same as layering. At least you try, nothing to loose, good luck!

Something else came to mind, since it's a roadside tree, you probably can expose some roots by digging, very lightly, about 2 feet out, [depend size] sometimes new shoots will grow there, then dig it up a year or two after.