Install Perian. It will generate thumbnails in Finder for non-native video containers such as AVI or MKV regardless of what your default video player is.
Try it anyway. It can be uninstalled easily from it's preference pane in System Preferences if you are not happy but it should generate thumbnails for you in Finder.
The developers have stated that there should be one last update for Mountain Lion but that was stated last year. They have been developing Perian in their spare time for years so there can be no complaints from anyone if it doesn't materialise.
Of course if you have Perian already installed ignore everything I just said.
The Perian creators could release the source code in the future for other developers to pick up the slack. Hopefully 10.9 doesn't make it completely obsolete. It's a testament to how good the program was in that there are no other all in one codec solutions for Mac that enable quicktime (and therefore finder video thumbnails) to recognise video files.