If the MKV files you have are 720p or less, a player form Google Play such as Dice Player or MX Player might be able to play the video file. These players enable support for files that are normally not supported by Android, but it depends on the files themselves as to how they play. Especially with high-quality or high-resolution files you might run into playback issues.
Another option is to convert your MKV files to a supported filetype, along with the proper settings to make them work properly on the Nexus 7.
This guide should be able to assist you with that: