Is the protocol described at https://tools.ietf.org/html/draft-ietf-secsh-agent-02 actually implemented by any ssh agent? pageant implements a different protocol as does OpenSSH's ssh-agent.
I believe the enterprise grade key agents from ssh.com utilise these scheme's inside there environments. (I have not been able to found a "proof" of that, just a educated guess based on the authors of the IFC Draft and the "hints" on the ssh.com product specs.)
the features the list seem to be able to do the things listed in the draft.