In theory what you probably could do (it is supported by BS.Player!) is
not play the DVD as DVD but as consecutive VOB-files (in playlist as indicated in
this post) and use external srt-subtitles. Unfortunately,
in practice, I'm pretty sure you will not be able to find suitable subtitles anywhere for such files. This simply as these text-based subtitles are not commonly made in a format/length which matches the VOB-files.