For those who is googling to this page: there isn’t a way yet. This is the same link as Automatically filling ChangeLog in the store. It’s a low priority feature so the snapcraft team will work on some higher priority works like bug fixes and etc.
Also, it isn’t important since the source codes itself is maintaining a version of changelog itself, like CHANGELOG.md recommended by most dvcs service providers or in-app changelog display (e.g. like Android application). Until there is a need and implemented an API or automated way to fill in, we have to fill it in manually.
For now, I will ignore the changelog from my side as filling manually in a duplicated way is wasting time. It’s better to redirect users to the maintained CHANGELOG.md site.
Not sure why I was being ignored entirely by the Snapcraft team since my last 2 posts. They used to be very responsive and helpful, even there is no good answer, they will acknowledge it happily and supportive as well.