Drill a hole into the top of the standpipe, shown by the yellow arrow in the pic. If I recall correctly, I drilled two 1/8" holes into the top of my standpipe, taking all of 15 seconds to silence my overflow.
Also, how did that much of your substrate end up in the overflow box to begin with? Or do you not have a box covering the standpipe? If you really wanted to get it out of the box, you could unscrew the standpipe underneath the tank and slide it up and out, then pour water into the box (with a bucket underneath the opening) to wash out the substrate. That way you don't end up with an incomplete seal on the bulkhead gasket.