Friction shifter for Rohloff speedhub?
The Rohloff shifting mechanism is designed specifically not to include a spring or other mechanism to move one of the directions. Since it pulls so much cable, it would indeed be difficult to do a single friction shifter. The 2-cable system is feature, not a bug, since Rohloff believes the 2-cable system to be more reliable than one cable + spring. 

They discuss spring mechanisms as prone to failure. I've had a high-end SRAM x.0 derailleur main spring break on me in the middle of a commute (fixed it with a zip tie in the middle of the cassette so I could at least get home with the 3 rings in front as my gears), so I've seen them go myself. The 2-cable system also makes it pretty simple to keep the bike going should one cable fail, since you can shift the bike with a wrench, something that wouldn't be possible if there were a spring trying to return the hub one direction or another (unless that were accounted for in some design).

That said, there is a trigger shifting option (ed. by cinq5) which is using 2 triggers. It's described in a blog post by Cycle Monkey.


  I love my Rohloff but I'm no fan of the grip shifter, only because it
  slips in my greasy hands. If it just had a different texture, I'd be happier. 


Best solution I've seen was to save a few of those giant rubber bands, that are used to tie broccoli in stores, and apply them to the shifter with a cross pattern for an easy friction increase. I need to go visit the produce section myself.