Do you have easy access to the top of the standpipe? Or is it obstructed with a grate or anti-vortex baffle?
For a short-term test and possible solution, you could just get 10' of corrugated plastic drain pipe and start a siphon from the pond and into the standpipe. You probably need to use wire or baling twine to tie a "J" into the bottom of the section of pipe that will be hanging in the standpipe so that air doesn't go up backwards against the flow and break your siphon.
(You also need to tie a 2x4 or something similar to your siphon pipe, so that there is zero chance it goes into your outlet pipe and gets stuck as a wadded up plug.)
You could also probably do it with 4" DWV pipe with to 90 degree elbows together at the end that goes into the standpipe to give you a turn-up. Put a screw cap on that end and a check valve on the pond end. Fill it with water, install the cap, and then hold it up high over the rim of the standpipe. Unscrew the cap, and as the siphon starts going, lower the siphon assembly into place and let it rip!
