I'm likely wrong, but I don't believe that FormStateClear('*') will produce the OP's desired result, at least I have not been successful in achieving his goal just now, in my admittedly-limited testing.
Specifically, FormStateClear() is a Magic function and, as such, can only be evaluated while the MDI is open. As a result, one may indeed clear the current state of all open windows, but the final position and size of the MDI will always be saved again afterward, even after the Main Program's Task Suffix is executed, when the user exits the application.
Moreover, child windows' positions, sizes, etc. are saved in the Application Data folder, while the position and size of the MDI is saved in the registry, as a matter of standard OS functionality.