I've heard of people doing something similar, but it's very tricky. The problem has to do with fonts and variable character sizes. If you use a fixed size font then you can test the length of the string to see how many characters will force you to print the XXXX and then use the Len() function in a formula to determine when to print the XXX's. If you are using a variable sized font then you just have to do a range and assume it won't be perfect.

To print the XXXXX's, I would create a second Details section with the XXXXX in it and then put that Len() formula in the Suppress property. That way it only prints if it meets the length criteria.