Hi Jimbo
We've used a few different top materials for customers in the past, Walnut, Koa, Brazilian & Cuban Mahogany, Myrtle and probably some I've forgotten but to be honest nothing really beats Spruce and Cedar.
Out of all the "weird" or should I say "non-standard" tops we've used, Mahogany is the only wood that stands a chance of being used as a soundboard for a "standard" model range but don't hold your breath, the wheels of change normally turn quite slowly
I could see us doing it in the future, maybe a special series or something like that.