Not sure if http could be used but you could do it by creating new users url with new realm (Authentication->Signing In), which has anonymous server as authentication. Then create new role, do not add any web resources. Go to UI options->Star Page section for this role, select "Custom page" enter your internal server's url. This way ssl vpn will work as proxy server and display your internal server front page. Every connection to this url will use license so make sure you have enough.