Many thanks for those two tips, Hans and Daniel. Really helpful! I never investigated what the 'offline application cache' or 'web app' tick boxes did, and I should have. Nor have I paid any attention to the general subject of Progressive Web Apps -- it looks like they will continue to develop in the wider world and they look like a good match for future Hype development.
Currently, as Hans says, Safari deprecates manifests, with other browsers taking an even more severe view of them, but I trust their support will last a few months longer.
I've had a quick go at creating a home-screen cached app using the method described here on a half-baked version of the project. It looks very promising. I agreed to the iPad request for me to release 50MB of local storage for the web app (the resources folder is 41MB in size, of which 39MB is a Quicktime movie), and the whole thing ran faultlessly in Airplane Mode except that the movie wouldn't load. I'll try shortening the movie's duration and compressing it with Handbrake and see what happens.
Thanks again.