Well, this is not really what I need. I'm seting up a real "network" of interconnected Moodles that have organized topology. We must control very easily who and through where a user would be able to roam. A user may not be allowed to roam to all servers and we would like a only sees the servers he is allowed to roam to and not all list.
I have accomplished this cloning the mnet_hosts block to a mnet_course_door block that allows an administrative configuration of the server jump tracks you may see in this course. the course is so used as a "moodlegate", only giving a mnet_course_door block with some litterature for passing the door, a physical space where enrolled users can find a way up to another server. I like very much this solution (works fairly well with the above patch), so we can design real browsing circuits through our Moodles that are very easily tailored for each user/role.
We tested the "course aware hop gate" wich works fine.
We should add a mechanism I did'nt found yet, to relocalize a user account so he could directly login on a sub-platform. I tried to login with a remotely transfered account, all SSO services enabled, and I could not login directly to the target platform with this account. Maybe I missed something.