I don't know the answer to your question, unfortunately, but I wanted
to recommend another tool as a workaround for now: Open Broadcaster
Software. Install with 'guix package -i obs'. I like it quite a bit.
obs didn't worked for me:
error: X Error: GLXBadFBConfig error: Failed to create OpenGL context. error: Failed to create context! error: device_create (GL) failed error: Failed to initialize video: Unspecified error
Looks like I need better drivers for my Radeon card.