HDR10+ is also stored in SEI messages. SEI messages aren't (necessarily) valid for a whole stream but can be attached to single GOPs and even frames. (Quite on the contrary, often SEIs
have to be repeated because they become invalid at e.g. a keyframe.) So you can put different HDR metadata into different parts of a movie.
x265 allows this but I have never done it myself.
http://x265.readthedocs.io/en/defaul...on-dhdr10-info