I was looking through the air suspension threads the other week, and one of the Dutch members (@dvdbob, I think) has had their gross vehicle weight increased after the instal of VB air. That was authorised and ticketed by the Dutch certification agency and seemed quite straightforward.
I had an hour to spare so waded through the UK equivalent which seems to be individual vehicle approval - IVA. It is all detailed in this manual (316 pages):
https://assets.publishing.service.g...oval-inspection-manual-passenger-vehicles.pdf
Section 44 is where it’s described.
The TLDR version: it seems it can be done, but it would be very tricky as a person presenting an air California would need to present ‘evidence’ that it can stand the extra plated weight. That would mean getting some paperwork from the air suspension manufacture of your choice I think. And then persuading the inspector at your local IVA centre that it was adequate.