So, the Surly Disc Trucker has chainstay IS mounts[1]. I have;

A rohloff hub with TS axle
An OEM2 mounting to go with;
A speedbone
An external gear mechanism

I'm thinking this is beyond even my Bodgit & Scarper(TM) skills. Is the
only option a long torque arm on a standard plate? Which looks like it
might get in the way of the external shifter.

[1]
https://bicyclehabitat.com/images/li...y-242015-1.jpg