The problem is, if you want dual-sport tires, there just aren't any that will fit a 15" wheel as far as I know... If you're sticking with street tires, the 15" is OK, but they might not fit in with the style of your build. I tried a few wheels on my CX, and I think the GL650 has the same issue - the final drive on the 650 is unique among all right-hand shaft drive Hondas - the wheels from the CX/GL500s and Goldwings won't fit at all. There are several wheels from left-hand shaft bikes that physically fit the CX/GL650 final drive, but the offset is wrong and the rim ends up about 3/4" off center to the left.
There are two solutions that I could find: you can use a spoked wheel from an '88 (i think?) Honda VT800, and have a custom rim made in a different diameter (17" or 18" for good selection in DS tires), or you can use the wheels from a CX650 Turbo, which gives you an 18" rear, and actually I think the look of the Turbo wheels might go well with your bike's steampunk style:
They're both kind of a PITA - the spoked wheel is easy but $$$$, and the Turbo wheel requires fabbing up a rear disc setup. And, unfortunately, the rear discs are NLA from Honda and spares might as well be made of gold, if
you can find one. Both of these wheels are also relatively rare since they were on one-year bikes. Hopefully if you go with the Turbo wheel you can find one with a disc still attached, that would make it cheaper.
It might be possible with some machining to make other wheels work, and you can also do some engine/frame work to graft a CX500 final drive on - not 100% of what all needs to be done for that. I know it involves taking the rear cover off the engine and swapping output shafts. Or maybe you could have a custom driveshaft made to adapt the CX500 final drive to the 650 swingarm. Either way you'd have to do some cutting and welding since the 650 final drive bolts on with 4 studs and the 500 final drive has 3. If you did that, you could use the CX500 rear wheels which are available in 16" and 18".