Seriously abusing authdata

[Elliot Lee]

The basic problem I'm trying to solve is a dialogue-free way for a party
to use Kerberos to prove that they said X (non-forgeable signature,
effectively).

What I tried doing was getting a ticket with krbtgt/REALM at REALM (typically
used as the first step in user-user auth), the authdata of which contained
the public half of a randomly generated RSA key. I know that the authdata
is making it into the ticket (since the ticket data size is directly
related to the authdata size). The main problem is being able to decode
that authdata on the other end (the "client" in user-user auth
terminology). In user-user auth, is there any way for the "client" to have
the KDC give it the auxiliary information from the TGT ticket that is 
normally used as second_ticket?

I'm perfectly aware that it's possible to write a public-key database
service or other involve-extra-parties solutions, but those are beyond my
ability to deploy.

-- Elliot