Hello,
Yes, the scenario you are describing is possible.
Instead of using modification rules, why not use the SRCVHOST argument of the wdisp/system_X parameter? The Configuring the Web Dispatcher for Multiple Systems - Clarifications and Examples - Application Server Infrastructure - … WIKI page will help you with this.
Anyway, if you want to continue using the modification rules, you should use HTTP_HOST no matter the protocol used. HTTP_HOST is related to the HTTP header "host", not to the protocol and hostname used at the internet browser.
Cheers!
Isaías
---------------
PS: made a small correction, the correct argument is "SRCVHOST" (there was a typo 
)