You can flush it to a good extent: Dribble some solvent like kerosene into the crack at the edge of the recess while spinning the freewheel. The solvent will come out the other side. Continue doing this until it sounds clean. The allow the thing to dry inside (this wil take long unless you take a very volatile solvent and flush it with that). When dry, dribble in some heavy oil into the same crack while spinning.