Earlier quoted context omitted.
Passwords should be hashed client-side anyway, so the real issue there is how to encode the password hash in a way that satisfies the legacy system. But this is usually plausible, e.g. by base64 or hex encoding the hash to avoid disallowed characters and then appending specific characters that satisfy the set of required characters.
Do you know of any systems that actually do this? It's the first I've heard of this technique.
It's a pointless technique as noted by others.
Along the same lines though is OPAQUE PAKE, where the server doesn't need to store a password hash, only a salt, making server compromise far less dangerous, it's quite elegant.
There's no implementations so far I believe.