Dynamically changeable database
You can use symbolic links with MySQL. All you would need to do is make a symbolic link for the .frm, .MYI and .MYD files of your MyISAM tables into the MySQL database schema directory that you want. After that, run flush tables; and they should show up.

It is also important that the MySQL user is able to access those files, so you might have a bit of an issue with user rights. 

Also, depending on your MySQL version, there maybe a setting in the my.cnf which enables/disables symbolic links, so you would need to keep an eye on that too.