If you can get the RS-232 commands for the Display I personally would not hesitate to make the connection. I think what their manual means by "Do Not Make Any Connection" is for some yahoo to plug something in and start tweaking with stuff he doesn't understand. I am a custom installer so I personally ignor all of those types of warnings. I stand by my previous recomendation though. I believe there is a full set of discrete commands available for that set. Which means that an IR setup is going to be the cheapest route to what you are looking for.
If you whant to see something cool that I built for one of these sets check out this link.

[Link: microcosman.com]

It is a QuickTime file so you may have to download the plugin but it's worth it.