No luck on the CX500 Comstar wheel. The drive flanges are the same bolt pattern but the hub itself is different and won't work with the 650 final drive.
So, it looks like my options are either to go with a CX650 Turbo 18" rear or get a custom rim for a VT800 spoked rear hub and offset it appropriately for the 650. Neither one would be a particularly cheap option, unfortunately. Still, even if I do either of those, my total cost for this bike is going to be less than $1500. I was hoping to keep it under $1K but I don't think that's going to happen.
I'm going to leave the rear wheel issue on the back burner for now and finish up the rest of the bike first.