This is something that can probably be improved, you are right.
Currently it just checks if a table can be SELECTed using that username and password. Perhaps if instead it tries to write a temporary value and remove it this would work better. Can you possibly take a few moments to file a bug against the mythbuntu project on launchpad regarding this? It will be a little bit until I can get to it.
Or even better, you can see the code used for the tester in ubiquity's mythbuntu_ui.py file. do_connection_test I think was the function I wrote.
If you want to improve upon it, and make it more robust, feel free