I think you may be over complicating it. Most CCTV overlays I have looked at before take data as if a printer, this is to provide compatibility with most POS software. This is why you won’t get a response and it’s the wrong method to use a PUT or POST like a web API because like you say it has HTTP headers and it will expect a response - this is standard behaviour and therefore you should not send data to it using this method.
Just set a print job to send what you want to display, what are you trying to do where that would not work?