Longstaffs in north Staffordshire make custom racks for their trikes. Robin Mather (sp?) also makes custom racks for his bicycles.
The cost, I suspect, will be astronomic! I'd expect a skilled fabrication company to charge waaaaay over £30 an hour for a job like that.
I bought a little pipe bender (8mm and 10mm) off eBay for a tennner and some aluminium hollow tube from B&Q to try and make a rack myself once. It didn't work, but it was a nice way to spend a weekend. Could you make up something yourself and take it to an engineering place and say "Braze that, please." ?