[stef25]

Isn't it possible to "burn" the subtitles on to the video file somehow, maybe with ffmpeg? So they're there permanently.

[worble]

Yes but it wouldn't be ideal since you wouldn't be able to turn them on and off, or switch between multiple subtitles.

[mperdemata]

yes, the container formats such as MP4 and Matroshka support this.

Also, it can be definitely done with ffmpeg.