Hi - welcome to the forums!

One thing you could do is retrofit a Brompton 2-speed rear derailleur to double up the hub's gears.

I think most 'off the shelf hubs' like the SRAM s7, nexus, SA 8 et al have a wider OLN spacing [ie need wider dropouts] making fitting them to a Brompton rear triangle nye-on prohibitive unless you know someone with a blowtorch that can make the necessary mods to the rear to accept the wider axle.

A lot of Brompton users double up their gears with a Steve Parry front derailleur install, which is probably a cheaper option than the Schlumpf Mountain or Speed drive, One can also consider the 'quick-and-dirty-ghetto option' of fitting a double chainring up front but no shifter, and then just getting oily fingers switching cogs manually when a different combo is needed for a big hill or whatever.

I do believe there are people who will do a Rohloff conversion though like you say - prohibitively costly to mere mortals.

EvilV on this forum has an SP derailleur - hopefully he'll chime in on this thread but you might want to search his posts to find out more about the mod.

Hope that's some help. Probably throws up more questions than answers, but 'them's the breaks'