OK, I just worked on this, following the documentation you linked to above. I am able to get to the main index of my other servers (TivoWebPlus on my various TiVos), but any redirects screw me up.
The way TWP works, if you click on a link that operates a separate module, it redirects you to the relevant address (i.e., clicking on "User Interface" takes you to 192.168.1.121/ui).
The problem with the way Apache seems to do reverse proxies is that this causes a problem (i.e., clicking on "User Interface" takes you to www.mydomain.com/ui
, which does not exist). Is there a way to tell Apache to stay on the servers behind the firewall? What use is this if it keeps pushing you back out to the public server?