There are obviously ways it can be "watermarked" easily, put some zero-width unicode characters in the output and you'll notice right away when it's copy and pasted.

But clearly, that can be stripped out easily by anyone who knows it's there.

This process, too, would seem to be easily reversible. Just have it run through another model and tell it to slightly reword it or rephrase it.

I don't think there is a technically solvable way of watermarking output like this.