I've given this some thought and it's definitely on the roadmap. A few other people have asked for this in the comments. Tagging people is very doable. Right now the model does a decent job with famous people, but not custom faces.
Seems like it's worth bumping this up in priority.
Is facial recognition (of known people) on the roadmap / a possibility?
It would be handy to be able to say photos of "mom" and it display all photos with mom in them. Probably not simple to build =p