Ted,
I was thinking about a similar problem - not because our ice is thick, but because it sometimes breaks up in the spring with 35 MPH winds from the south and the ice beats the h*ll out of anything in the water.
My "poor boy" solution was similar to your pipe through the clay for that section of the pipe run. However, I was going to have a vertical pipe in the pond, then a threaded elbow, and then a valve in the horizontal leg.
I can control the water level at "normal pool" by having a 24" vertical pipe. If I want to pull the pond down a foot, then I shut the valve, and swap the vertical pipe out for a 12" vertical.
Before winter, just shut the valve, remove your vertical pipe, and screw in a plug (to keep your threads clean in the elbow).
With a proper slope on your horizontal run, all of your drainage pipe will only contain air during the winter for the entire section downstream from your valve.
It should be easy to customize as needed to fit your exact needs in your pond.
*** I have not yet installed this system at any of the little ponds at our farm. Any comments pointing out an obvious stupidity in my design would be greatly appreciated prior to installation.
