That is a predicament, and a huge reason that I hate hate putting any kind of elbow or bend in an overflow pipe, but thats neither here nor there now, I would start first by siphoning down the water level with the 60 ft of pipe that you already have that you were sticking up the outside end, that will achieve two things, with all the water pressure pushing against the obstruction from the inside getting it to release or back out will be virtually impossible.
What size pipe is that, go purchase a couple glue on fittings for it, possibly 22.5 degree fittings or 45 degree, then take about 15ft of that pipe, glue a check valve on the deep end, and stick it down the inside of the dam into the water, glue a fitting on the top end that will go flat across the pond dam, then another elbow fitting, then the rest of the pipe down the backside of the dam with a valve on the end of it, should reach way lower then the elevation of the inside piece that went down into the water, thus giving it a siphon ability.
In the middle of the pipe going across your dam, at the highest point, cut the pipe and glue a tee with the third leg sticking straight up, into the upright glue a short section of pipe with a screw on cap.
Now with the outside valve closed and the check valve on the inside end holding the water in the pipe under the pond surface you can remove that cap and fill the whole pipe full of water with a bucket , when it gets completely full, close the lid and go down the backside of the dam to the end of the pipe and open that valve, that should start your siphon. Let it siphon down well below your overflow tube inlet, that will give you a little freeboard before the next rain event, even if it does fill back up with rain before you get your pipe unplugged, getting the siphon going again will be simple.
Then you are ready to tackle and deal with your obstruction, and I am not sure how I would start with that one, I would probably lean toward the idea you had of capping the outlet end and putting an orifice in the cap to blow back up the pipe to blow it back, but rather then compressed air I would be inclined to pump water backwards into the pipe with some kind of pressure pump, that way when you do blow it backward a few inches to get it loose you can keep pumping and float it to the top of the pipe so you can reach and retrieve the stick.
Lastly, if you do succeed in clearing it, look up Agri-drain drop inlet screens to install on top of your pipe to avoid any future problems.
Good Luck! let us know if you have any questions about the jibberish trying to explain how to build your siphon, I think there was a sketch of it on this forum some time in the past, you may be able to use the search function and find it, it will definitely help you visualize it. and it does work like a million bucks, have lowered several ponds with it to do some work on them.
