One of the Harmony benefits is that the database often lists commands that aren't on the actual device remote. ie often the device remote has one button (to save space on the remote) to cycle through the TV inputs, but the harmony database will have a range of commands, one discrete command per input. If you were forced to learn your commands from the original remote you would never get access to these other commands.
Often in there are power toggle commands etc that could do what you are after?
Also, didn't the TIVO not go into standby or something, which used a lot of power but made it always on and ready to go etc?