Is there a hole in the 'eccentric' part? If there is, stick a small allen wrench in that hole and rotate the cranks around until they push/pull the eccentric by hitting that allen wrench, then hold the crank there to keep it under tension while you tighten the pinch bolts.
I'm not sure how clear my description was, but basically you can use the crank arm against an object stuck into that hole to gain leverage.