I've taken a look at duplex, and it looks perfect for what it was designed for, i.e. emulating the standard printer options.
Anyway, the command I was actually looking for was:
Code:
pstops '4:-3L@.7(21cm,0)+0L@.7(21cm,14.85cm),1L@.7(21cm,0)+-2L@.7(21cm,14.85cm)'
…which I eventually found in the pstops manpage!
The page sizes aren't quire right (they're for Letter paper) but that's easily fixed…