Single Speed owners who want gearing options that don't require a derailleur may choose to "dingle" (or is it "twingle"?).
Dingling involves riding two rear wheels, necessitating a front fork wide enough to accomodate the second rear- the Salsa Fargo V2 fork, for instance.
Then there's the Shimano Alfine internally- geared hub, available as an 8 or 11-speed, thumb-shifter and cable the only requirements. The Alfine's a bit hefty, weighing in at over 1.6 kg for the 8 (1.7 for the 11), but aren't 8 (or 11) gears better than 1, particularly in an event such as the Tour Divide, with its significant elevation changes, long, flat stretches, and so on?
Such a question naturally elicits nearly as many views as there are riders. See here, too, for Tour Divide finisher Cjell Money's views on the relative merits of singling and dingling (though not Alfingling). His blog is all over the course, so the relevant sections may be a little hard to locate, but a peruse of the whole post provides no little entertainment.
Of course, the above modifications come at the cost of simplicityand weight and produce a machine that ceases to be single-speed.
At present I am disposed to leaving the dingling and Alfingling to others.