Disabled pushbuttons: In my application I display several pushbuttons (e.g. P1 P2 P3) each of which executes an InputScript. The user needs to click on P1 then P2 then P3 in order to complete the process. I would like only one to be active at any given time, and the others shown as inactive. Is this possible?

Yes, this is possible. Use the "-disabled" option in the pushbutton definition:

Pushbutton ....     -disabled