I'm running my former 1x9, then SS, then fixed commuter, now fixie mtb Karate Monkey.
I have a Boone cog drilled with out for a disc brake rotor mount. I reversed the rear while and do not run a rear brake obviously.
I run a pretty tough gear to get a lot of momentum for climbs.
I've tackled the local regular ride which has some technical challenges. The more epic ride areas with bigger climbs have seriously taxed my fitness, but that's part of the fun.
I'm still working on log crossing, ledges and the like. The big wheels really help you roll over things easier. I run 165mm crankarms which also help clear obstacles.
If riding a SS is a full body workout, riding a fixie off-road is a Navy Seal workout. I've been riding fixie for several years, so the learning curve was not too steep.
Have fun and laugh when everyone calls you nuts!
Here's a pic from the commuter set-up with rack.