A couple of options - make that field read only for the main window and then create a windowless action that pops a window with that field on it. Modify the process to have this new action at the spots you want and then you can add extra actions like adding notes, reminders etc as you want.
As an alternative you could make the field auditable and then make sure people know that they can change it, but the fact they did will be recorded and what changes were made
Another option will be: Create a windowless action (combined with a window - like dave said) and then create a conditon in the process like
"action XYZ has be done?" if yes show the action(s) "Add Note" etc.
Or create a condition based on a calculation and the calculation will check if it's date "2011-12-20" if true then show action "Add note"
Hope this helps