I have checked this again and thought it through. As the broker needs to know the username on whose behalf to operate, you probably have to add the --auth-id cmdline switch:Please use
x2goclient --broker-noauth --broker-url=<URL>
For this to work, you have to set the check-credentials config option x2gobroker.conf to false.
x2goclient --auth-id=<broker-user> --broker-noauth --broker-url=<URL>
This is ok with X2Go Client installations on some local/private machines. On thin clients (with no local login) this does not work as the thin client is not aware of the person's username sitting in front of the TC.
Hmmm...
The other option would be to automatize the SSH login once the user has authenticated against the broker. This feature is already implemented in X2Go Client but needs some extra work in the public X2Go Session Broker.
Another issue, I have to address later: The current broker implementations in X2GoClient expects that the broker user ID is identical with the SSH user ID (i.e. the X2Go user ID) of the targetting servers. This is suboptimal, IMHO.