...But the scenarios you want are already there, no...? Have you set up your audio and video pipes either in the Setup Wizard or the "A/V Devices" section of the web admin? If you have those set up right, all you should need to do is select the auto-generated scenario you want (watch live TV, playback a movie, watch a DVD, play PlayStation) and LinuxMCE will automatically turn the right things on and set the inputs on all of them. If you have lighting control, that's usually taken care of automatically, also. I use this all the time with my DT360 - it's sweet.
If the TV and whatever the MD is connected to aren't on, I believe you can turn them on by activating the Orbiter menu from an IR remote. If not, we can work on adding a "Display On" button (there is already a command for that - you can see it on the orbiter).
The tricky part is to get the remote to trigger those scenarios, since that sort of thing isn't in the usual remote setup.