[imp] redirect of virtual domains to certified ssl domain?

[Fabian Ruff]

Hello,

i'm actually trying to set up imp for a
webserver/mailserver with several virtual domains.
I thought of providing access to imp through
one certified https adress.
To have full namespaces in each domain i set
up the imap accounts like username_domain_com.
I now want to redirect requests on webmail.virtual.com
to https://webmail.certified.com and pass imp
the virtual host which was originally requested.

For usabilty reasons users should just type in their
username, the virtualhost is then appended by imp.
I took a closer look on the $conf['hooks']['vinfo']
thing but I think this place is to late for SERVER_NAME
like you suggested in my case. I have to do
the redirect before the login if I don't want the Browser
to complain about an untrusted certificate.
I thought of writing a small index.php for
webmail.virtual.com parse the SERVER_NAME an then redirect to
https:...

My question is, how to pass/register/whatever a variable
which can be evaluated (seen)by the vinfo function. I tried
finding it out myself, but i'm tired and dizzy from all the includes
and redirections in IMP.
I also want this variable to survive a "Log out", so that another
user from the same virtual host can log in.
I could simply register a session variable but i'm not sure
if this is the cleanest way. Maybee there is better place for
it in the dark, scary corners of horde/imp?

Greetings,
Fabian

P.S:Impressive piece of software, really!