Use Brompton BWR 3-speed hub to create a full 700c wheel?
A Brompton rear hub has a non-standard over-locknut distance of 111 mm, I think. A single-speed bike probably has 120 mm. So you'd need to cold-set the frame.

Sturmey-Archer (which I believe makes the Brompton hub) also makes a 5-speed hub that has about the range you want, although from what I can tell, the OLD on that is longer than your rear triangle's spacing. So again, you'd need to cold-set the frame, although less violently. Their other 3-speeds should fit 120-mm rear spacing.