On these changers are they able to report what dvd they are looking at and send some sort of id back across the rs232 port?

I think I've found a good way to use a dvd changer with mythtv.

I believe there are video hubs on the market that have priority switching based on what port. So the dvd player gets the highest priority. When you click play dvd in Mythtv it tells the dvd player to power up and begin playing disc x. You would control the dvd through myth as normal using the remote but the switch is sending the dvd signal to the tv since it has the highest priority. When you stop watching the movie and power the dvd changer off then mythtv is displayed again.