No. It can be deactivated but it is not an Assignable controller.

All of your Assignable Controllers can be seen as follows...
From the Home screen:
Press [EDIT]
Touch “Control” > “Control Number”
From this screen you can see what each of your Assignable Controllers are set to do for this Performance... the ones in the lower right corner in black background are global... you can reach them by touching the shortcut box to “MIDI Settings”.

Can the audition button be told to play a specific key note in a drum part?

Well, actually yes, sort of... the [AUDITION] button can playback an Audition Phrase. You can create your own Audition phrases and keep them in the MODX “SONG” Folder as .MID Files. Any data in your Song Folder can be associated with any User Performance - it will playback at the tempo stored with the Performance.

From the HOME screen:
Press [EDIT]
Touch “General”
You can select an Audition Phrase for the current Performance. In the lower right corner touch “Update User Auditions”— this will make the contents of your SONG Folder available for Audition.
Change the Audition Bank = User and select your File by name.

Extra Credit:
Each time you load a .mid File it remains in your MODX SONG Folder. (You only ever have to load a Song once).
Press [UTILITY]
Touch “Contents” > “Data Utility” > tap the “Song” Folder to view the contents.

Audition Phrases can be triggered by simply touching the [AUDITION] button.

Thanks. I’ll give it a try.