Within NoteWorthy, Alt+BS works as Undo in single line edit controls, but not in multi line edit controls. For example, in the Info Command, Alt+BS will undo anything except Comments.
In looking at MSDN's About Edit Controls, I see several messages dedicated to processing Alt+BS. NoteWorthy behaves exactly like 'Windows Address Book' (wab.exe) which was the only standard windows app I could find that uses a multi line edit control. NotePad and WordPad support Alt+BS in the main edit window. It seems that they do some extra message processing to achieve this.