Give me a couple of weeks and I'll look into the input switching in Hijack - probably not the all-encompassing solution that you proposed, but possibly to give a config.ini option that configures Hijack to switch to aux-in whilst the mute line is active and switches back to the previous input when it goes low. I can't see why you;d want the dimmer line to do anything.

The volume adjustment is already handled by the Volboost_AUX parameter.

For other digital inputs, wait to see if Patrick's tuner kit project goes through. He's discussed making other digital inputs available there.