At the risk of offending the Brompton faithful, I've been browsing C H White's
Dahon Spares website and have found a cheap(ish) source of Pantour hubs. I'd contemplated them before, but ordering from the USA was prohibitively expensive, both in terms of cost price and subsequent additions (customs duty and shipping).
However, here are
Pantours for £40 - at 74mm, Brompton width, but with only 14 holes. Could I build this into a 28 hole rim, missing out every other spoke hole? 14 spokes seems remarkably few and I can see everything collapsing at the sight of a pothole in the road. I'm aware that Pantours are not famed for longevity but this would seem a relatively inexpensive way of giving them a go.