App store add support to install apps on custom Firmware using the mobile app, I think you get my point
The ideal situation is for the custom firmware community to fork the Flipper app then make a pull request so the feature can be added to the official apps. I do like the idea though.
You can install apps on custom firmware if it comes with supported (release) API version. The problem you facing is that custom firmware authors prefer merging our dev branch for their releases and thus you end up with unsupported (yet) API version.