Creating a web service accessible to a regular ignition client sounds like a fine approach to me. There’s no way to run a headless client that does whatever you’re thinking it would do.
Edit: I think you might be able to use a gateway tag change script or a 7.7-style tag change script to watch a tag for the ‘request’, but I don’t know how easy doing the file manipulation and whatever else is needed from Ignition’s scripting system will be.