How to grant access to a user's session manager by another user?

[Alessandro Vesely]

Hi list,
I reckon this is a faq, but I'm unable to find a response.

Scenario: (debian squeeze/gnome)
I run startx at a virtual terminal, logged in as a user.  Next, from
an xterm window I execute "su <another user>", typically root, in
order to gain access to different files or commands.  For using X,
I run sksu, sux, or the "root terminal" instead, or setup XAUTHORITY
and/or ICEAUTHORITY as needed, but even then I cannot connect to the
dbus session.

Although a dbus-daemons are running for each user, I get an error from
gconftool-2 if I try to connect to it as the other user.  I'd guess
that's because the other user, even if it's root, either is not
authorized or doesn't know how to find the SM.

Question:
What is the proper way to configure access to a given session?

I've noticed various policies in /usr/share/dbus-1/services, but they
are for specific applications that require admin privileges.  I don't
want to grant more privileges to my personal user-id, I want to allow
other local "users" to share my session.  Any hint?

TIA