There are green screen plugins for that. You could chain Snapcamera's virtual cam output into OBS and use OBS 's chromakey filter to underlay whatever other content you like. Y I u can then output OBS to another virtual cam output for use in Zoom or wherever else via the Virtualcam plugin for OBS.
It'd be really nice if someone would make a nice OpenCV virtualcam output so one could drive all this from a (relatively) simple Python script.