Is there any reason to be forced to use a fake .srt extension? (Seems a bug.)
If we had language code subpages, we could use the translate extension right now: subpages are always clean enough if there's no <language/> or other similar stuff, see the text on https://meta.wikimedia.org/w/index.php?title=Fundraising_2012/Translation/Poongothai_video_%28captions%29/de&action=edit
Note that people do expect subtitles to work with our translation tools, see e.g. https://lwn.net/Articles/527081/