[purpleman]

I'd be interested in testing, for sure. I know it's probably asking too much but it would be great if you could find a way to have subtitles add-ons from Kodi working from within MPC-BE. I ask because you talk about "communication between Kodi and MPC-BE", thus raising my hopes.

Edit: feel free to contact me by PM for test builds, help, discussion, whatever really. I'd like to contribute as much as I can.    

I think that a dramatically easier approach would be to make it so that the subtitle feature of Kodi (and any plugins you have installed) are accessible from the context menu within Kodi (before playback starts) rather than from within the video player.
I'll look into this.    

Yeah, that's another option, for sure. I honestly don't know whether that would entail modification to all subtitles add-ons code or just the portion of Kodi that handles the process. On the other hand, thinking about it, that means starting playback to check whether subs are present or not, and if they are good or not. If they are not, stopping playback, searching for subs, restarting playback.

Meanwhile, I've done some experiments with EventGhost (a programmable remote software solution). While launching MPC-HC as an external player, I managed to get Kodi to open the subtitle search window, while MPC-HC is paused. It works and it downloads subs. There's some video corruption outside the subtitles search windows but everything works perfectly.
The problem is going back to MPC-HC and having it use the subs you downloaded. So far I've only managed to get there using the "Reload file" functionality, in combination with the "remember position" functionality so that the file is reloaded from where it was paused before searching for subs.

Edit: and I've actually managed to configure EventGhost to handle everything in a pretty seamless way. Thanks for the inspiration!  

That's great. Might be a good idea to share your exact EventGhost configuration (screenshots or otherwise) so that other forum members could try the same.
I still plan on compiling those custom versions for playback resume.