If somebody is willing to make PRs to the repo I can test & approve them, but I think that it would be better to transfer the snap to the snapcrafters group, refine it, and then try to upstream it.
And actually I had success! It did cost my some time and a few struggles, but now it is updated.
The latest snapcraft config which has now been built is at https://github.com/phw/musicbrainz-picard-snap/commits/master . I changed the setup to make use of the now available kde-neon extension, integrated appstream metadata support and updated the dependencies for what is needed by current Picard.
We’ll have a new release soon, then I’ll also update the website to link to the snap.