It can be done with a script, but it's not very secure.
Here's one possible way of doing it. Use this code as the custom Validation script of one such field:
event.rc = (app.response("Enter the password to edit this field:", "", "")=="1234");
If the user doesn't enter "1234" when exiting the field the value they entered into it will be replaced by the field's original value. If you want to apply the same code to multiple fields I would recommend putting it in a doc-level function and then just call it from the fields you want to protect. That will make editing the password or text of the message later on much easier.