The CX650 drum brake didn't work with the VT800 wheel, so I ordered a VT800 brake. It fits perfectly:
Gotta love Honda parts-bin engineering.
I have proper wheel bearings on the way to match the VT800 wheel with the CX650 rear axle, so the only remaining issue with the rear wheel is that the drum brake plate has a 20mm hole. I need it to be 17mm. But looking at parts fiches, it looks like there is a bushing pressed into that hole that rides on the axle. The 650 brake plate is the same. I should be able to press out the bushings and swap them. Then I'll be ready to send the wheel off to Buchanan to have them lace a new rim to it.
Would it be better to have a 17" rear or an 18" rear? I'm not really sure. Most of the tires I'm looking at are available in either diameter.