I think I might be leaning towards an mp- command rather than a new endpoint. But there are some pros and cons.
The pros for a new endpoint that is at the same url as the GET for the data is that someone who hasn't built Micropub or wants to use an external service for their stories would make that possible. By de-coupling it, it provides more flexibility.
The cons for a new endpoint is that the authentication layer, the set up of a new endpoint can be a technical hurdle that might prevent more people form getting involved. For those that already have a Micropub endpoint, the addition of the stories feature becomes a pretty simple add on. It also means there's a new communication layer that people would have to understand, whereas Micropub with an mp- command is pretty simply understood.
Either solution would be pretty simple for me based on my set up, but I'm trying to think what would be more difficult for: a) Wordpress Users b) Static Site Users, anyone else that is building their own tech stack it's probably not a huge difference.
That said, I use an external Micropub library in my Node.js server that does the authentication against my token endpoint automatically. So if I were to add a new endpoint, I would have to write the code to handle the verification of my token endpoint. While my token endpoint is on the same server, for other people in similar situations, they might have juggling the authentication process.