Handling for non-TV "channels"

Looking at the draft, one of the potential use cases for this spec seems to be headed in the direction
of allowing TV OSDs and controlling (e.g. the entire TV UI) with web technology.

It seems like the spec allows the users to implement most of that, but is missing handling of non-tuning
source, such as external input "channels". (e.g. HDMI1/2/3)

Not sure what would be the best way to retrofit that into the draft design as it stands (adding known
external source types in TVSourceType for known available input formats could in theory work, although
not extremely clean), but wanted to raise the issue for discussion first.

Sangwhan