I'm no professional on the subject but a 12 ft tall concrete wall would need some support to hold it against the force of 12 ft of water I would think, which I guess if you just banked good dirt up halfway or better on the backside would create a lot of support for the wall, and the bottom half is where most of the pressure would be, that would allow you to get a little closer to the property line.

Than there would be the problem of access to the dam end of the pond, I guess you could build a walkway over the top of the wall but you will also have the issue of the depth right inside the wall. if something were to fall into it or something, you would need to swim all the way to the end of the wall to get out, or build some kind of escape stairs or ladder in several places to accommodate an accidental slip and fall.

It probably could be done, just going to need some planning, also there is the dirt that comes out of the pond on the inside to make a bowl for a deeper pond that would hold a lot more water, you'd need a place to go with that. just a wall across the end of the existing topo probably wouldn't hold a very big volume of water.

All the really good ideas I've ever had came to me while I was milking a cow.