I haven't tried this exact combo, but I have tried something like it: I was trying to see how short I could make the stem so that my wife could borrow my bike. The bike has an MCFK 6 degree bar and the shortest I have been able to run is a 90mm 6 degree stem (RaceFace Turbine). I tried an 85mm Loaded AmXC stem with 5 degree bend but that was 1mm too short to allow the bars to not interfere with the upper clamp and end-cap of the Lefty.
If I remember right the Flatforce comes in max 77mm length, and the problem is exacerbated by the angle being steep so that the bar is less far from the fork leg for a given stem length.
So until they come out with the longer Flatforce stems that they have promised I don't think this combo will work... unless you got one of thesebars
that bend forwards significantly and might clear the fork leg (from the pics it looks like the bar bends forwards about 15mm or so whereas my bar bends back about 3mm.... so maybe you gain 18mm or so of clearance which might just be enough... but it'll be close and you'd have to try it and see)