Subclass Standard Common dialog(OCX) and get full control on it. You can change text in any buttons/labels/textboxes, make them invisible or disable. When user perform any action with common dialog, your app receive notyfication and you can proceed/cancel this action, prompt user with msgboxes etc, etc.
Add following functionality: intercept Open/Save dialogs toolbar buttons events, intercept context menu in filebox, changing dialog size, move/size controls in dialog.