No one commented on this, so I contacted Surly for an expert answer.

The third hole on the left dropout (the one that protrudes from the dropout body, much like a regular rack/fender mount boss) is indeed there specifically for the addition of a Rohloff hub, which like all IGH must be anchored to the bike frame in some way.

Rohloff has at least 5 solutions to this problem, depending on the bike. One use a torque arm attached to the CS, another uses an arm between the external gear mechanism and ISO disc brake mount, another uses a plate with raised tab which engages a slot on a special dropout which the frame must integrate into its construction, another uses a slotted plate which engages the ISO disc mount hardware, and yet another uses the same slotted plate which engages a specially located bolt in the dropout.

The third hole in the left dropout of the Disc Trucker is a receptacle for the OEM2 Rohloff plate, diagrammed below.

This is a neat solution for one-half of the Rohloff retrofit problems. The other still remains - chain tension - which is solved by either an eccentric BB, horizontal dropouts, or a rear-derailleur-like Rohloff chain tensioner.

http://www.rohloff.de/en/products/sp...ain_tensioner/

And finally, you still have to contend with routing two full-length housings from the left dropout to Rohloff's unigue gear shifter, which is not drop handlebar mountable without modification or special parts.