If there is a registry setting then there should be a variable available, or the setting must be readable. I have to assume that the registry value is just there so that the information survives between MC sessions and over reboots. I also assume that there is a way to look up all the fields/variables in the database, but I don't know what it is. Whether reading the value requires programming or not I have no idea.
If there is a variable available you could just program a button to change channel to whatever is in that variable, using the simple 30002 MCC_PLAY_TV command, or in your case as you are on a client, perhaps the 30022 MCC_PLAY_TV_CHANNEL_FOR_CLIENT command.
All you can do is try these commands, unless someone comes along and gives you the answer. I'm afraid that isn't going to be me. All I can do is guess.
Note: I did try to set up the "Live TV" button on my remote to start live TV on the same channel as I was previously watching, but I was told that while this used to be possible, it was no longer possible. See
http://yabb.jriver.com/interact/index.php?topic=82413.msg561896#msg561896 The previous channel was no longer the default when Live TV was started, or more correctly a "0" was passed to the command and that no longer meant view the previous channel. You could try changing to channel "0" and see if it works now.
I didn't pursue it much after that, and just start Live TV on the first channel in my list. But that leads me to believe that the previous channel is recorded, it just isn't used as the default any more.