Wish Bromptons were 135mm rear axle.
I would just use a spare dual drive hub I got for next to nothing.
I understand the Scottish Brompton Dealer in Glasgow does custom modifications..
Alfine hubs and such..
a Caprio Cassette, dual drive would be a clever product..
high is a 9t.. (otoh 12 from the 2 speed hub can go on the 2 x3 hub ..
though the 13-15 combo half-steps better, )