No, unfortunately, you can't customize Smart Controls layouts. I really hope that in the future we'll be able to completely customize layouts, that would be a great addition to this feature!
Only in some layouts, by entering your label text in the inspector:
To change the master layout, choose Track > Show Output Track, select the Stereo Out track, and change its Track layout.
It will show the Master only if the selected track's output is set to Stereo Out. Otherwise it displays the Smart Controls for the Aux that the track is routed to.