[imp] multiple installations, single login point

[rick at havokmon.com]

Ok,  I have a totally non-standard question :)

I have users who have different access levels.  I will be giving some of these
users access to different parts of Horde system.  Everyone will get IMP, but
only some will get Turba, and some will only have access to certain parts of IMP
(Say Identities).

I have these levels configured within MySQL, and I was planning on the following:

1.  There will be multiple installs of Horde, 3 at this time.  I REALLY like the
new CVS, and only some users will get access to that version.  Others will get
the main release.  Some will get Turba, others won't.  I think it'll be easier
to hard configure seperate installs, than to try and wedge authentication INTO
Horde, and keep it somewhat updatable.

2. A single login page for all users.  

I assume
$conf['user']['alternate_login'] = false;  
will point to my special login screen.  

That way, nobody will 'cheat'.  

But what's the best way to deal with the redirect.php?  The custom_login.php
example loads Horde classes, but I don't want to tempt version conflicts by just
loading one version's classes.
Should I just have a single custom_login.php for each version of Horde, that
just does a javascript submit onload.  That custom_login would be called by, and
passed auth info from, my main login.php.

Does that sound feasible?  Is there a better way to do it?  Is there some sort
of central access-level in Horde that I've missed that might work here?

I've been here on and off, and I can't believe that Horde has improved so much.  
Thanks!

Rick