If the index (usually stored in the last couple of chunks of the movie file) is complete, you can preview all the downloaded parts.
You can also watch all the downloaded parts if the index is completely missing, since BSplayer is able to rebuild it. The only case in which you'll not be able to watch the downloaded parts is when only the final section of the index has been downloaded, since BSplayer thinks to have it complete, and doesn't rebuild it.
I never used external plugins for eMule, but I always watched the preview of all the partially downloaded files with BSplayer. Now a lot of time has passed since last time I used eMule, but I doubt it stopped working like this in latest builds.
Maybe a feature to rebuild the index can be implemented in next BSplayer releases, this way if the downloaded file has not a complete index, you can choose to rebuild it and watch the entire completed parts without problems
