Axle hell

[64Chevelle]

I would probably just machine the register further so that it is substantially (at least .100") undersized and then press fit a sleeve over it and remachine it the way you now know you should have to begin with. Any sort of loose spacer for the current dimensions is going to be so thin that machining it and maintaining any kind of roundness and/or wall thickness tolerance will be a chore IMHO. You may be able to machine it to a "standard" size and then use an off-the-shelf hubcentric ring as well.

That idea struck me as probably the best solution. I've got a hold of some thin flat shims that I'll cut and bend in fitting pieces to temporarily center the brake rotor.

Depending on the axles, I've also seen a few that were machined to accept bolt-on axle registers. The axles were center tapped for custom machined billet aluminum registers and then retained with a single fastener in the center...I believe it was so that they could easily swap between two different wheel patterns with different wheel registers. Those combined with screw-in studs seemed to work slick.

That would be awesome

Considering how hard most of us work to balance our wheels and tires, the closer your rotor is to running true to center the better.

Agreed, I believe the high weight of the rotor also makes small imbalances quite noticeable.