View .mkv in quicklook
A quick search turned up QLVideo, which I've now installed. It provides limited functionality with mkv (as well as avi, flv, asf, and a few others) including preview screen shots in QuickLook and thumbnails in Finder's GetInfo window. Unfortunately it does not actually play the video.

https://github.com/Marginal/QLVideo/releases/tag/rel-172

Sadly, Perian (a brilliant piece of software that I miss dearly) does not work with Apple's new replacement for the QuickTime infrastructure in MacOS.