Is there any way I can specify for a particular application to never be hidden when I press Windows Key+D or click show desktop in Windows 7?
There is no easy way to do this or an application that provides the functionality you need. The direction i would take is writing a small AutoIT script with a infinite loop making the window constantly active.
*This example will keep the window class "Notepad" continually active even if the show desktop button is clicked WinActivate focuses on the window specified. I have added a sleep of 100MS so as not to use 100% CPU usage.
Depending on the language used to create the application that would not hide on "show desktop" one could hook the *form_resize* event (In VB6 Language) and if triggered show the application window as this is triggered on win+d or "Show Desktop".
I would just create a small script in Autoit (the easiest solution).
EDIT: As "Bavi_H" suggested, one could also continually set the "Window" state as "Restored" by replacing the following