I can't help but notice that metadata for MKA's is not retained across different copies of MC that aren't in a client/server setup. This can mostly be worked around via the Fill Properties from Filename... window, but it's sort of inconvenient to have to re-enter everything.
I wasn't aware of MKA files. I'm unsure about sidecar support, and of course, I know nothing about tagging support in MKA. But I can talk in general about transferring metadata from one installation of MC to another.
As you said, if they aren't in some sort of client server arrangement, you have to rely upon files in some way. If the metadata in the files isn't there, then you need something else. In my opinion, the cleanest MC based solution is to export and import an MPL playlist. MPL is a subset of XML used by MC to store pretty much every field MC holds about an particular file. MC can read and write these files, so they would seem to be ideal for copying metadata between MC installations. As long as your file paths on both systems are the same, you're all set.
Just highlight some files, then do File > Export Playlist and select MPL as the type. Then import that playlist on the other system and it should update all of the fields. There's some odd behavior, that in some cases, you need to import the MPL file *twice* before it reads the values. I can't remember why or under what conditions, but if you have trouble, try importing a second time.
Maybe this will help you out.
Brian.