MPTagThat supports external scripts, written in a .Net language. The CSScript engine is used for that. Scripts must reside in the Scripts folder and are compiled on their first usage.
The currently active script is selected in the combo box on the ribbon and pressing the Ribbon Button or Alt+E applies the script on the selected files. The sample script, which comes with MPTagThat switches the Artist Name: e.g. John Fogerty becomes Fogerty, John