The filler text is not editable as it's just displaying what text will be filled in there when the data is sufficient.
That is odd. It sounds like a formula field or a DB field.
Edit: I'm wondering if it would just be easier to put 2 small text boxes on the outside of the desired field and have them anchored to it, so they would always shrink and/or expand to the size of the field as needed.
I'm not sure if I understand what you mean.