Just a suggestion, but... there are a number of $20-$40 "USB-only" media players out there that deal with MKV and PGS subtitles just fine. And a $60 budget will find ones with network interfaces to pull from a media server.
How often are you actually PLAYING a BD?
Blu-ray player w/ USB for MKV playback?
Re: Blu-ray player w/ USB for MKV playback?
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How to aid in finding the answer to your problem: Activating Debug Logging