Just to keep everybody up to date - the Sheffield fabricator hasn't got back to me, so I'm assuming that avenue is effectively closed. However I've been in touch with Matt's man in Hampshire, Graham Bates and he seems to think it's all perfectly doable, which is very encouraging.
Andrew (spud) has very kindly sent me a VX top rail and bottom rail (which I believe to be common across all variants), and I've found an i.e. rail in my random collection of parts, so these three pieces of rusty metal will be heading off to GB Engineering some time next week. I've requested that the bore of the main flange be increased from 30mm to 40mm, which will comfortably accomodate an in-head thermostat and the 3mm bleed hole that Guy Croft recommends.
Graham reckons that the best way to proceed will be to have a base spec top rail, with extra cost options available to suit. Base spec will be VX top rail with the secondary pipe deleted (in other words, using in-head stat and remove external stat from plumbing entirely). For those who wish to keep the external stat (whether for appearance or by preference), the secondary outlet will be an option, as will the sensor bosses and bracket for the i.e. variant.
I'm assuming that everyone will require the extra VX inlet/outlet on the top rail? Is there any requirement for the original carbed variant with the unmolested top rail? I can't see the point myself, but if needed it should be the cheapest of the lot. Anyway, once I get an indication of a likely price, I'll let you know.