That subtitle file naming convention is used by Kodi. Plex use something very similar. I haven't looked at the Kodi wiki but I believe forced subtitles would actually be named;
Episode 01.en.forced.srt OR Episode 01.english.forced.srt
from what I have read. That is how Kodi and Plex set subtitle flags for external subtitles; by structuring the subtitle file name.
As per the JRiver Wiki and my example, MC does not seem to use structured subtitle file names to indicate language and type. i.e. Forced or Default. To use an external subtitle file with MC, it should be named;
Episode 01*.srt Where * is any identifier you want to use, but not a flag such as "forced" or "default".
If someone knows different, please speak up.
However, Matt did say back in the release notes of 20.0.138
19.0.138 (6/5/2014)
8. Changed: Sidecar subtitles will have a higher priority if the name of the sidecar exactly matches the name of the file.
So if you name the subtitle file exactly "Episode 01.srt", it should be given priority.
I am still looking into using external srt files, and how to flag them as forced English subtitles. I think I have deleted the examples I have had previously. Can't find them.
But for you, as you are using MKV files anyway, and you now have srt files of the forced English subtitles only, from the site I linked to above, why don't you just use MKVmerge to merge the srt subtitles into the video (NOT re-encode the video), and flag them as English and Forced during the merge.
Here are some very good instructions on how to do that:
http://madaboutbrighton.net/articles/encode-forced-subtitles-using-mkvmergeThere instructions are for using the command line, but I believe it can be done through the GUI as well.