As I remember, when together with Gimli, we worked on "AirHockey" the DMD with miniplayfild function, there was some problem of "move to of bam" on the backbox, at that time, so I don't know if it is possible now.....Having said this, I understand that you would like to move or disappear a second DMD that you use for your "Tweaker" in the backbox, right? well if you don't have any other solutions, there might be one:
Build a 3d model based on your backbox, leave windows for the primary dmd and for your dmd, in your dmd create a model as large as the window and place it in front, with bam (as everyone knows, I hope) you can make it invisible a model via the texture, and with the "SetTexture" function via script commands when making the model appear yes or no making the dmd visible or not.
The biggest difficulty in coordinating the current texture of the backbox to the model and window of your "DMDTweaker" , but it is solvable, a matter of time.
One thing I don't understand, if it's for those who use the cabinet, why put your "DMDTweaker" in the backbox? you could insert it into the Apron, this way it is easier to make it appear and disappear via an invisible texture.