Following this discussion, it was eventually decided to leverage npmjs package repository to publish Joplin plugins. Then a script is used to fetch these plugins and move them to the official plugin repository:
This repository will then be used to load the list of plugins in the main app, and install and update them directly.
If you are a plugin developer, please consider running
yo joplin --update from the root of your repository, and let me know if you notice any issue. Once that's done, you should be able to run
npm publish, which will put the package on npmjs. There's some additional info about it here. If you have any question about it or if the update doesn't work please let me know.