If I recall the protocol-extension design correctly, such a setting could only be set at session start, which could be annoying --- at the very least we'd have to tolerate entries for unrecognized data types, since clients couldn't be expected to have checked the list against the current server in advance.
The protocol extension design has the drawback that it can only be set at startup.
What if we were to allow changes to the setting after startup if the client passed the cancel key as a unique identifier that only the driver would know?
