@ebr: Excellent! Using the Unknown type works great. I hadn't properly interpretted the utility of it before your post.
I've created a .mkvst (st stands for subtitles :geek: ) extension in classes_root that's otherwise an exact copy of .mkv so it has perceived type etc. set correctly. Media Browser has no trouble finding and playing the files.
In the configurator I set Unknown to open MPCHC and all is well. Perfect solution. Thanks.
To aid anyone else in doing this I'm attaching the text of the reg file installer contents. I couldn't find an extension the board would let me upload (failed on no extension, .txt, .reg, several others...).
Use notepad to copy everything below this line starting with Windows Registry... into a text file then rename it .reg and doubleclick. Should do for you.
Windows Registry Editor Version 5.00