I can think of a way to "fake" it, but I haven't tested the idea.
You would have to create an alternate tuning in your selected key and load it through a tuning SysEx. In your example, you would create a 7-note scale, eliminating C♯, D♯, F, G♯, A♯. The keyboard layout would be disrupted... Playing a chromatic scale would produce a diatonic scale in the key of G.
When programming the unison, you would hold down three keys separated by apparent whole steps.