We have not documented the different ways to send in Metadata to XProtect, but there are alternatives you could consider:
Act like an ONVIF device, where only Metadata streams are supported. You could support e.g. 8 streams on same ‘device’ if that make sense for you. The protocol specification is public.
Take a look at our VPS (Video-Processing-Service) library – this is for Linux development with GPU’s. The XProtect driver sends video frames to your service and get metadata back to XProtect. If your platform is for Linux – this may be an option.