Chris, is your pipe PVC also?
If so, what about merely slipping the elbow over the pipe without gluing it? Depending on how snug it fit would determine how much leakage you will have. Then, if you were worried about it freezing, you could remove the elbow in the winter and draw down the water to the normal overflow level.
One of my ponds has two drain pipes, one for lower water in the summer, and a higher setting to make sure the concrete deck is fully covered in wintertime, to protect the top from popping off when it freezes. To raise the level in winter, I just slip a plug over the end of the pipe. It trickles some, but not enough to draw down the water level. It freezes UNDER the ice every year, no problem. I don't know what might happen if it were encased in the ice itself.
