You can set up the global property:
CMS.CMSOutputFilter.OutputFilter.CacheItems
The current value is "username;sitename;lang;browser" but you can add the domain there like: "username;sitename;lang;browser;domain"
You could add the custom code to for example: ~\App_Code\Global\CMS\CMSApplication.cs (the AfterApplicationStart method).
-hg-