I thought it was doable. However, after looking into it I've learned that the API for this is quite limited. For Microsoft Store apps, context-menu entries are only shown if the app is the default for the given file extension.
This means that if I would add support for that, Mp3tag needs to be set as the default app for, e.g., *.mp3 files — and I think this is a little bit over the top. While I certainly love my app, I think the default app for any given audio file should be the player.
I've cast my vote at this Windows Developer UserVoice proposal and hope that this will be improved with a future version of Windows. Till then, I'll probably continue collecting negative ratings on the Microsoft Store due to missing support for the Explorer context-menu.