For non-fixed songs find the longest non-tutorial song and use that for picking mods. Something isn't quite right with ScreenEdit though (it doesn't display existing attacks so you can add attacks but you cannot remove them, global mods can be modified normally).

stepmania/src/ScreenOptionsEditCourseEntry.h

@@ -26,6 +26,7 @@ protected:
 	virtual void ProcessMenuStart( const InputEventPlus &input );
 
 	CourseEntry m_Original;	// use this to revert when cancelling
+	Song *m_pLongSong;
 	ThemeMetric<bool> SHOW_MODS_ROW;
 };