1) Replace the current edit control for new messages?
If you mean replace the text edit control used in GroupWise compose view dialogs then no, the forms designer will not do this. The form designer lets you create completely new event driven forms. It is not designed to replace elements within existing GroupWise forms and dialogs.
2) allow additional code to be run as a user types into a control.
Code could be executed as a result of changes detected in a control element appearing on a new Formativ form, but not within an existing GroupWise form or dialog.
If my understanding is correct, you wish to automatically save GroupWise draft messages periodically as a user enters text using the standard GroupWise email compose views. Unfortunately the forms designer will not help you with this requirement, nor will Formativ in general. Such a low-level feature would need to be built directly into the GroupWise client by Novell.