Looks like it is impossible to get User instances or something similar to User.get_full_name() directly from a template.
Why is there only strings of usernames stored in Page instead of foreign keys?
It's because Pages can be created by non-users, scripts for example. But I agree it would be a nice-to-have.
Will close as a wontfix for now, as I don't see a strong enough reason to break this and make it more complicated.