Python hit this issue in the last few years after the transition to unicode-everything. Since the filesystem is supposed to have an encoding, but doesn't enforce it, Python needed to choose what to do with a filename that contained bytes that were invalid for the encoding. In order to make a round trip of decoding and encoding result in the same bytes, they decided to use certain surrogates to represent the invalid bytes:

https://www.python.org/dev/peps/pep-0383/

So with most systems being UTF8, this means that the encoding is "UTF8, except with bytes that would be invalid UTF8 mapping to this range of surrogates". So you've got an encoding with no holes, which is compatible with all valid UTF8 (since those surrogates are invalid UTF8).

It's not common, so it doesn't match that criterion of yours,but I think it's a great solution and hope it catches on. We need a solution for "text, except with graceful fallback when someone decided to put arbitrary bytes in there"