foo_exec would simply execute an external program (with parameters if needed)
I see a simple config page where you have a list and you add your commands + title of that commands
e.g.
Clean File [ cleanme.exe -r %__filename% %artist% ]
MP3Gain [ mp3gain.exe -whatever %__filename% ]
Then you have this menu option called 'Execute Command...' where you find the list of your commands that you created.
Of course you could add any TAGZ in the parameters ... I think it could be quite cool. Think of the nice possibilities with the good old .bat
nIRV