Add a small IR blaster in front of the TV that defaults to jamming it. Now the channel up/down buttons will no longer have any effect on the TV and can be used to control your custom app. If some other button is pressed that you do want the TV to act on, momentarily stop the jamming in order to replay the command.
I actually thought of ideas similar to hijacking the remote control signals. The problem is that this project is intended for mass deployment and work on arbitrary TVs with a digital receiver. I'd be happy to do this maneuver for my parents, but it's not practical for the purpose of helping less tech savvy people fight government's media dictatorship.
Basically, here are the constraints
- user must be able to access PRESET internet streams and turn on the TV using the factory remote control
- must not require customization for different TV brands/models.