You would need a device driver which operates at Ring 0.
Ok, in that case, I guess I am after the minimum device driver that's sole duty is to switch from ring 3 to ring 0 - callable from any ordinary app.
That would be enough to prove the concept of allowing me to create my own mini-Win64 that purely uses memory above 4 GiB.
Note that I am only interested in executing non-buggy programs, so I don't mind OS/2 crashing the same way that MSDOS crashed if you run a buggy program.
I simply debug the application.
Running everything privileged has worked fine for me for decades. I'm not trying to make a commercial quality product. Fleshing that out is left as an exercise for the reader or whoever sees a commercial opportunity for something more robust.