channel: ['stories']
or you could make a specialized Micropub app like that have a config screen where you define what additional metadata it sends and then people could use whatever method they wanted, a channel property, a tag property, etc. I definitely don't think a new attribute would be right, and I know channel isn't really used anywhere except your and my sites, but we also can't do tag as a default or else the posts would automatically show up on people's websites as a normal post (unless they configured posts with a specific tag to be routed somewhere else). So this is tricky, but I definitely think something like this would be required for me to use it routinely. My default for my site would probably be to use a 'stories' channel and then to be able to set an audience on each photo/video so that some things I could provide to a subset of people.