George, you are correct in that Roin machines their 17" Campi clones from a single billet of aluminum.
For what it's worth, I contacted David Besharat (Mr Fiat) a few months ago and asked him about the possibility of having 18" and 19" wheels produced. Mr Fiat's wheels are made by Roin. Here's the response I got:
"Our supply responded today:
I can only consider to produce 18" because 19" is out of range of billet provider.
Of course if [you] will be interest of 18" Pantera I can find a solutions to modification original drawing.
Which model of Pantera wheels are interest and which size?"
Based on his response, I would say 18" is possible but if you only want one set, you're probably going to have to deal with Mr Fiat. They likely have more clout with Roin, so minimum quantities may not apply.