And indeed my use case for this has instances of both cases: originally decoded using ASCII and the non-ascii bytes must end up as replaced characters, and originally decoded using utf-8.

I'm also not sure that it is worth adding this.  If you know what you are doing the solution is obvious, and if you don't know what you are doing you shouldn't be using surrogateescape in the first place :)

Now, if there were or there is intended to be a more efficient C level implementation, that answer might be different.