Not sure about cross-platform, but for macOS I started this plugin by creating a minimal virtual camera: https://github.com/johnboiles/coremediaio-dal-minimal-exampl...

@seanchas116 made a Swift port of my minimal example https://github.com/seanchas116/SimpleDALPlugin

Overall it _was_ very difficult! Apple's documentation and sample code for CoreMediaIO DAL (virtual camera) plugins are terrible. I just brute forced it for hours trying all sorts of different combinations of things before I got something to work.