This is might be trivial for someone who actually have a one, but after countless hours reading and watching about the Rohloff speedhub I can't answer this question myself.
AFAIK the OEM grip shifter is not the most loved feature of the system. It can't be used with drop handlebars, - as all other grip shifters - it is slippery when wet, and it is not a trigger shifter. So far I have found 23 quirky ways to overcome the handlebar issue, and one trigger shifter review.
My first idea would be just to use a one-cable (maybe indexed) friction shifter, and some counter pull mechanism on the other end of the cable to replace the second cable. Seeing the how much effort has gone into mounting or replacing the original shifter, there should be something why this won't work.
So why? Why there is no friction shifter available for this hub?