As I mentioned, I've been using an FO reference (Lovel, FO Developer's Handbook), so I'm aware of the blank-or-not-blank property.
In fact, the DITA OT already includes conditional code using this property. Looking at the code, it appeared that what I had to do was create a static-content region with the "intentionally left blank" message, and modify the simple page masters of the "-last" pages to use that.
But it does not work for me. It appears that there is more to it than that.
In another location, I saw a mention that more extensive code is required to make this work. I'm trying to find out what that additional code needs to do.
Bob