As for repurposing a frame custom built for a Rohloff, I see no reason why another hub gearbox cannot be fitted. In the standard dropout/exccentric bottom bracket model common on Thorns it may be a little more difficult than on the standard Rohloff-sesigned sliders as for instance on my Utopia, where a different frame end is simply fitted to the sliders and then provides chain tensio., but I don't see that it is a big deal.
As Il Padrone says, using a Rohloff specific frame for something else is sub-optimal, but I reckon a Shimano hub gear on a Thorn is pretty neat, and much better than using a frame meant for derailleurs. Even Thorn do this - they sold off some of the Raven Sports Tour frames with Shimano hubs.
I fitted a Shimano Nexus 8 Premium to my second Thorn Raven Tour frame. Only issue is the cable run. Its possible to use one of the two cable runs for Rohloff that run along the top tube and reposition the entry on the Shimano hub to aim up the seatstay. I didn't do this, as the only way to fit a Chainglider to the Nexus 8 Premium needs the cable parallel to the chainstay.
Compared to my Rohloff equipped Raven Tour, the one with the Shimano hub has two empty cable runs and 1 cable run held by zip ties. I have the zip ties anyway on my Rohloff equipped bike, as I needed them for the rear lamp (Thorn recommend using a battery rear lamp, but I wanted two of my 3 rear lamps to run off the hub generator).