Does an MKV tag/metadata editor for Mac OSX exist?

The best tool you can get for this task is mmg as part of the mkvtoolnix package:

http://www.bunkus.org/videotools/mkvtoolnix/

It contains a header-editor where you can change the 'tags'. It is available cross-platform, the link you are most interested in should be http://www.bunkus.org/videotools/mkvtoolnix/downloads.html#macosx.