Eccentric rear axle would be good. Might be a bit advanced for me I’m afraid! I guess you would machine the outer profile of a normal axle (which would be hard enough to get accurate for press-fit bearings), then put a 10mm drill in the chuck and mount the axle where the tool sits, and shift it off centre a suitable amount?
I assume a 10mm through axle, as standard QR wouldn’t grip it hard enough?
Would you get enough adjustment to take up chain slack? The max it could be off by would be about 2mm, as the axle is 15mm thick in the freehub, so wall thickness is only 2mm.
If you used a standard QR the centre hole would only be 5mm so you’d have more adjustment, but drilling a 5mm hole 135mm long 😕 Would that work?