Twisted's web server does something kinda-sorta like what you're looking to do. The general approach used is to have a loadable python file define an object of a specific name in the loaded module's global namespace. Upon loading the module, the server checks for this object, makes sure that it derives from the proper type (and hence has the needed interface) then uses it to handle the requested URL. In your case, the same approach would probably work pretty well.
Upon seeing a command name, import the module on the fly (check the built-in import function's documentation for how to do this), look for an instance of "command", and then use it to parse your argument list, do the processing, and return the result code.
There likely wouldn't be much need to pre-process the directory on startup though you certainly could do this if you prefer it to on-the-fly loading.