The cocking dogs (levers) don't extend out past the knuckle of the action on most Greener designs.
What you can try is this, though I'm a little hesitant to recommend it; remove the forearm from the barrels, and hold it onto the knuckle of the action in the position it would normally be in when the gun is closed. Hold it firmly against the knuckle, and rotate it downwards. This takes a considerable bit of strength, since you no longer have the leverage of the barrels, and it's easy to slip. Rotate it downward until you hear the 'click' of the tumblers going into place. Then never drop your hammers with the barrels off ever, ever again.
You could also try taking the cover plate off; remove the screw and knock the plate out from above with a narrow piece of dowel ( requires a very thin blade, properly fitting screwdriver ). This should give you enough access to the lockwork to get things re-cocked.