[horde] Apache Proxy with Horde/Imp?

[Josh Trutwin]

Has anyone setup an Apache Proxy configuration similar to what I'm 
trying to do below?

I manage a server that hosts about 100 domains and for obvious reasons 
I only want to maintain one single installation of horde/imp.  I 
currently have it setup such that if someone from domainx.com wants to 
access webmail they use a URL:

http://mail.domainx.com/

I have Apache using mod_rewrite to redirect this to my domain:

http://www.mydomain.com/horde

<VirtualHost x.y.z.x:80>
     ServerName mail.domainx.com
     RewriteEngine On
     RewriteRule   ^.*$  http://www.mydomain.com/horde 
[T=application/x-httpd-php]
</VirtualHost>

I would rather have this be done via a proxy setup using something like:

<VirtualHost x.y.z.x:80>
     ServerName mail.domainx.com
     ProxyPass / http://www.mydomain.com/horde/
     ProxyPassReverse / http://www.mydomain.com/horde/
</VirtualHost>

This gets me close, when I pull up:

http://mail.domainx.com/

I see the Horde login page (with no images for some reason) but the 
login form has an ACTION tag set to:

<form name="imp_login" 
action="http://www.mydomain.com/horde/imp/redirect.php" method="post" 
target="_parent">

So when they submit it shows mydomain.com in the browser instead of 
their mail.domainx.com domain.

This looks like it's getting generated by the following in imp's 
login.php script:

$formAction = Horde::url('redirect.php', false, -1, true);

This isn't a HUGE deal, but of these 100 domains I manage, some of 
them are my clients, and some are my partner's who runs a seperate 
hosting/design business and they don't know who mydomain.com is.

Thanks for any help,

Josh