How to route shifter cables along frame to a Rohloff external gear mech?
As you have a steel frame, you will get the best result if you have a competent framebuilder add the appropriate brazeons. There are two sorts of brazeons that one could use:


Cable tie guides, where the cable housing is fixed in a groove with a small cable tie (looks much better than a big cable tie around the entire tube!)
various shapes of short tubes or rings, where the cable housing is fed through


Have a look at Ceeway for what is available (or talk to your framebuilder).

For a visually pleasing end result, the frame will need a new paint job after adding brazeons.