The "right" way to solve it is for everyone to use seekable MP4 (or seekable WebM), especially since things like Chromecast don't natively support anything else

The "right" way to solve it is for everyone to use seekable MP4 (or seekable WebM), especially since things like Chromecast don't natively support anything else. That's hard to do but possible if you provide a highly desirable service (people will start to complain for torrents in unsupported formats).