Ha. That's really cute.
This falls completely flat in some cases.
I am using the Store version of the media player "foobar2000". It is able to associate to the file types it declares in its manifest perfectly fine. Neat.
Here's the thing about fb2k, it's very heavily plugin/component oriented. That is, it is possible to add support for more file types not shown in the manifest. Sure, in some cases they can declare support for the "*" type, but this is dependent on the application developer being aware of this and choosing to do it. Some choose not to pollute the Open With dialog by default. That's fine.
Here's what's not fine. I want to be able to associate the ".vgz" extension to foobar. Unfortunately, it chooses not to declare the "*" type, in the name of being a good neighbor. As such, while I can enter the .vgz extension into the incredibly hackneyed Default Apps settings that you've been tricked into liking, foobar2000 will never show up in the menu. I can't choose to "Find another app on your PC" or whatever because it's a Store app and does not appear in the file manager. Even if I were to unhide hidden files and track down the exe (I did) and associate it this way, if any other UWP app tries to open a .vgz file they will encounter an Access Denied error because the exe is inside the WindowsApps safety bubble. Of course, Explorer (and most regular Win32 apps) opens the associated file fine. Windows Terminal won't. The "Files" app from the store won't. I use Files as my go-to file manager (it has tabs) so the inability for Files to open these files fundamentally breaks my use case.
I looked a little further into this and dug into the registry that I've been repeatedly told I "can't" use to make this modification. I examined a functional UWP app->file association. I looked at foobar's. Immediately I noticed that Windows had created a regular Win32-type file association record for foobar, but a far more complex definition was used for the working UWP app. The vital registry entry, in this case, is "DelegateExecutable" (inside one of the shell -> open -> command keys). It is a string value containing the CLSID of the UWP app to open. After finding foobar's CLSID I was able to amend its association record to use the UWP style and, lo and behold I just set a file association via the registry despite being told this was impossible! Better still, this has caused foobar to appear in the Open With dialog at all times and it creates the right association type, just what I wanted!
So, please, tell me. What exactly is the supported way to accomplish what I needed here? I suspect you can't tell me. :)
All I needed was a "Show all UWP apps" checkbox or something like that on the Open With dialog. This could have all been avoided.