I was going to type a comment similar to gehajake's. I believe he has proposed probably the best solution.
However, I think you need to be certain you have correctly identified the problem. I like that your pond goes down 4" very quickly when full and then the water level drop more slowly. Can you confirm that behavior?
Put an easily-read measuring stick in the pond and note the daily water drop after the pond is full. If the pond drops the first 4" very quickly and then slows under nearly identical evaporation conditions, then I suspect you have a pipe seepage leak very high in the face of the dam. That is absolutely the best place to have it!
As to your top seal solutions: Clay actually expands in contact with water, as does bentonite (a specific type of clay). Unfortunately, these clays then shrink (and crack) when they are desiccated during exposure to air. Soil floc (and its near cousins) are types of cross-linked polymers that also swell when exposed to water. However, I do NOT know if they subsequently shrink after long-term exposure to air. (You might need to send a PM to TJ?) If his product also shrinks, then I don't think you can create a "top seal" at your drain pipe with those types of materials since they will all crack after multiple wet/dry cycles.
Concrete does not have that property, but I still think it would be a poor material to create a top seal - without some additional labor. Dry cement mixture does expand as it is hydrating with water, but it then begins contracting slightly as it cures. Therefore, your sealed concrete around the pipe may still let a tiny bit of water through after it cures. However, the good thing about concrete is that it is erosion proof in your application. Once compacted clay and soil materials develop a tiny water flow path, it tends to get bigger over time as a few particles lining the path eventually get dislodged and washed away. Concrete does NOT suffer from that particular problem.
I might try a cheap fix first? Dig out the dam face with shovels as deeply as you can along the pipe. Then use wood (or well-compacted) clay to create your concrete form. I would then rough up the pipe and spread the heavy black mastic they use to seal concrete water pipe (bell and lip). If you can't find that, use the black roofing mastic with embedded fibers. Spread your sealer to ensure 360 degree coverage plus coverage for all of the concrete thickness around the length of the pipe plus an extra inch that will stick out into the air. Then pour your concrete "collar" at the top of the drain pipe. After the concrete has cured, use a putty knife to push the exposed mastic down into the pipe/concrete circumferential joint.
(If you do have some membrane type material similar to a pond liner, you could even line the bottom of your "form" with a sheet fitted tight to the pipe like a traditional anti-seep collar.)
If that concrete "top seal" doesn't work, then I would go to gehajake's solution and expose a much longer section of the outlet pipe with a mini-excavator and try creating a good seal across a larger length of the troubled area.
