another easy way could be to it with mod_rewrite in i.e. the 00_master.conf
I assume you've modified the 00_master.conf to use ssl already 
 
i'd create a file sslproxy.map below /etc/apache2/
with content like this : 
Code:
non-ssl-domain.tld         http://www.non-ssl-domain.tld
 
so you can add here a lot of domains one under the other - where the first entry is the alias used for something like 
https://admin.yourdomain.tld/sslproxy/no...omain.tld/ 
the create the sslproxy directory below the document_root of the used vhost (in my example it would be /var/www/ispcp/gui/sslproxy )
and add the following at the end of your 00_master.conf 
Code:
RewriteEngine   on
RewriteLog      /var/log/apache2/sslproxy.log
RewriteMap      sslproxy                   txt:/etc/apache2/sslproxy.map
RewriteRule     ^/sslproxy/(.*)$        ${sslproxy:$1} [P]
 
of course you need to load mod_proxy to make it work.
(a2enmod mod_proxy) 
then test.
Pls be aware that most cookie based auth methods will fail because its the wrong domain in the cookie - so the browser won't return something to the server.  Also all paths should be relative because otherwise you've mixed content you won't like ... and which will be shown the user as