OK, I've heard back from everyone I've contacted about this. The news is that 22mm is an odd size within the automotive industry! Maserati wheel nut sizes are shared with the Vauxhall Insignia and the Range Rover and that's about it.
I have been unable to find anything in either chrome or black. The only option I can see available is the dark grey (which is for the Insignia!) Range Rovers have stainless nut covers, but the shape is wrong and wouldn't fit the Maserati. They are also expensive at c.£2-£3 each.
I'm happy to negotiate on grey if anyone is interested and will continue to search for chrome versions, but this seems unlikely as I have contacted both suppliers and manufacturers.
If there's not enough interest for a group buy of grey then have a look on ebay for Insignia wheel nut covers in 22mm, there are quite a few on there.