This is quite a popular application for Joseph Kuosac mid riser bar on Bromptons.

You got to keep the cables unchaged (can be a little bit longer for the perfectionist), a lower riding postion (might not be as low as S type) and a very large handlebar (can be 600mm fulll length uncut) for cockpit.

BTW, Joseph Kuosac also has carbon midriser handlebar and the soon to be released carbon ergonomic S handlebar too

This one is from older M to mid-riser Joseph Kuosac uncut