I have been trying to get this to work as well.
I am using a logitech harmony remote, and I've tried the folowing 2 things;
1. Start WMC PC in RecordedTV - this causes WMC to start in WMC RecordedTV.
2. after starting WMC PC, send command to change input to "RecordedTV" -nothing happens.
If when running WMC, from either of these 2 options, I press the "RecordedTV" button on remote it does go into RecordedTVHD, so it works there, so not sure why 2nd option doesn't work, maybe I need to try adding a delay before changing input.
I've added it as an option in an unreleased beta I'm working on. It does take about 7 seconds between Windows Media Center startup and startup of Recorded TV HD because Media Center delays the loading of background addins.