Many of the subtitles come in formats different than SRT. Converting them manually can be a tedious task (and a never-ending one).
Hence a proposal: minidlna should recognize MDVD and MPL2 subtitles in .txt and .sub files, and convert them to SRT on the fly. This a relatively easy and inexpensive operation, and converted subtitles could be cached in... well... cache directory.
MDVD is a frame-based format, so it's neccessary to query video file for FPS.
Unfortunatelly I lack knowledge of C so I'm unable to submit code for this. Otherwise I would.