If the wheel centers are welded to hoops as you say, the issue is finding a supplier of 18" and/or larger hoops which should not be an issue in the US. I could provide centers or designs that could be cut and welded at the correct offsets. Again the problem is not making a larger wheel, its making it look fairly correct as the DIA. grows. You can not just blow it up---the center cap becomes to large and the lug areas grow out of range. It diameter growth issue sounds easier than it is.
Kirk, Roin wheels are one piece; machined from huge billets, rather than a billet center welded to an outer hoop. There are a few wheel manufacturers in the USA who will custom make two piece wheels as you suggest.