When you say +pipewire you mean just audio playback? If you are pushing video to a picture-in-picture overlay a user might expect that so yea you could write to the pipewire socket like any other program. It's usually fully open for you to do just that.

I use v4l2 regularly with OBS. In order for Chrome/Chromium to see it you need to make the device before launching Chrome/Chromium. You can start v4l2 devices automatically by setting a modprobe config for your kernel.

My v4l2 notes might be helpful http://technex.us/2022/06/v4l2-notes-for-linux/