Raven with ex box?

[humedavid]

How difficult would it be to change the cable routing and run them under the bottom bracket and along the chain stay.

There's a halfway option there as well, along the top tube, down the seat tube and along the chainstay, an S shape.
This is effectively the cable run on my Airnimal folder, though the top tube is lower.  It works fine as long as the curves are gentle, sometimes after folding I have to pull the cables into shape again.  it would have no practical advantage, though IMO it could be made to look neater.
For attaching cables without braze ons, there's a variety of options including Rohloffs own
https://www.sjscycles.co.uk/hub-spares/rohloff-cable-manager-kit-5201/
I've also seen bare inner wires run down the downtube with cable stops at the top and bottom (Not on a Thorn)
https://www.sjscycles.co.uk/gear-spares/1-12-problem-solvers-backstop-clamp-on-cnc-twin-slotted-double-down-tube-cable-stop/

PH thank you for the links, great stuff! I've been looking for a cable tidy option, but couldn't find the one from Rohloff!

[Pavel]

I run the ex box on my Raven RST, much like Brummie does, except I think I have the cables running one position more inline with the rear stays.  The Ex box is really the way to go.
I have no problem with it and my rear tubus Cosmo rack.  I can't remember for sure just now, but I believe that I had the Thorn rear rack mounted with it at first when I got the bike.

[Andre Jute]

I have no problem with [the EXT box] and my rear tubus Cosmo rack.  I can't remember for sure just now, but I believe that I had the Thorn rear rack mounted with it at first when I got the bike.

Tubus in any event sells an extender kit for its racks, consisting of some punched flat plates and some nuts and bolts. Or, if you baulk at the usual Tubus overcharge, you can make up your own kit from punched stainless steel plates from your local hardware store. Or raid your grandson's Mechano... You will certainly need something with the Cosmo in particular and presumably with many other racks too if you go from 26in wheels to 700C or some other larger diameter, because so many racks appear to be designed for 26in wheels.