Why the limitation on conditional formatting? Will this be published to the web? Are you using Filemaker 12?
The simplest option to employ uses conditional formatting but does not require a triger field as your current field can serve as the "trigger".
Type Add description here as layout text and put it on top of or beneath your text field.
Use this conditional format: Not IsEmpty ( Yourtable::YourField ) to change the font size to 500 to make it disappear if data is entered into the text field.
To do the same without using a conditional format, you can define a calculation field that produces the same result:
IF ( Not IsEmpty ( textField ) ; "Add description here..." )
If you use the calculation field, make sure that it is transparent if you put it on top of the field and make sure that its field behavior settings deny access to the field in both browse and find modes.
No limitaion to conditional formatting...it was the only solution I could think of.
Thanks for the solution!