I would try using water under pressure from the pond side to flush out the pipe.
I agree with Scott. A hydraulic "drill" (high-volume water-flow through a 3/4" PVC pipe, connected via hose to a centrifugal trash-pump) on the pond-side of the clog would be my approach.
Connect (glued) and insert into the drain-conduit enough 10' sticks of PVC pipe to contact the site of blockage. Then couple the pipe's terminal end to a centrifugal trash-pump (fed with pond water) and let the water-jet blast its way through the blockage.
