I wouldn't use barrel nuts at all. I would take a hanger bolt like this: I would drill a slightly oversized hole into the neck heel, chop the hanger bolt to length, and thread it in, locked down with epoxy. The epoxy will allow us to use an oversized hole so as not to put any significant compressive stress on the neck (we wouldn't want to risk a stress fracture, either during installation or 100 years from now). It would still "thread" into the neck, but it would be a loose threading and the epoxy will permanently lock it in place. Then I would use a nut inside the guitar body. Inside the body, I would use a spherical washer so that the bolt alignment/neck angle is not critical. I think this is a very unconventional arraignment and would probably have luthiers worldwide looking at us with scorn, but aside from that I think it's very simple and workable.
Screwing the neck into the body is a worthy endeavor. But...
NEVER fool yourself that what you've just invented hasn't been done before
in the guitar world. There's almost a thousand years of "prior art" in the stringed instrument universe. There's very little low-tech guitar luthiery advances you can think of that haven't been done previously--a dozen times.
Regardless, guitar-improvement hubris affects us all at one time or another. It affected me when I devised the barrel bolt system--a reaction to my teacher Gurian's pinned mortise and tenon--which was a reaction to Martin's tapered dovetail--which was a reaction to the joint-less Spanish Method.
You're free to react to my barrel bolt system--be my guest! You won't get any flak from me, or any luthiers for that matter--they don't know any better. But you will get flack from formal cabinetmakers. My old friend Ian Kirby, instructor at the Royal College of Cabinetmaking told me when I brought up the subject, that one of the Ten Commandments of woodworking is "never screw into end grain." HE would look upon your idea as inelegant and hazard-prone.
But not me. Because his field is cabinets. Because I know that screwing necks on has been a luthier tactic since luthiers learned to cast lead or iron screws and bolts. Lag screws appear on lutes made during the Late Middle Ages and Renaissance. Taylor Guitars uses cap-head machine screws that screw into wood-threaded inserts inserted into the end-grain of the heel. Most luthiery schools teach two neck joints, tapered dovetail and neck screws. Period.
When I devised the barrel bolt system, advisedly, I never announced or claimed first use. And guess what? I had been using it for years when I discovered that my old partner, Michael Millard of Froggy Bottom Guitar fame, had devised it quite independently on his own for several years before also. Exactly the same arrangement. For exactly the same reasons:
1- the pinned mortise and tenon system was difficult, tedious and complex. As is the tapered dovetail.
2- we hated the look of two unartful nuts on the headblock staring at you from the soundhole.
3- we were concerned about the long-term effects of screwing into end grain, or the short-term effects of threading a screw into the endgrain heel.
4- you could get the barrel bolts at the local hardware store. You don't have to turn your own wooden taper pins.
5- having to change the neck to body angle throws bending stresses onto the screws, which throws strain onto the wooden parts. The rotating barrel in the barrel-bolt system solves that problem.
I confess, your selection of the spherical washer as a solution to 5) is a clever hardware choice. And your idea of drilling an oversize hole in the heel and casting epoxy threads didn't occur to us, but you'd have to be pretty confident that the entire epoxy plug wouldn't just pull out someday.
Your wanting to invent your own system has a noble heritage, so I won't be the one to squelch it. I'd experiment with how much force is required to pull out the epoxy plug first. Then go ahead--be my guest. Build your own guitar with a personal neck-attachment system that you think best fits your own priorities.