I have seen very big trees growing in very shallow soil and I am betting so have you. Many of our natives make a living that way, where abouts are you? Roots can go a long way sideways and depending on the tree can work their way through cracks in the rock. I would pile the mound and mulch out as wide from the tree as you can.
How is the drainage and what type of rock? Avocado would be one I wouldn't try but I think a heap of others could work if drainage is ok.
My grandfather is on a similar very shallow top soil over shale and has very established macadamia, citrus, annona, bananas etc. The drainage is good though. The macadamia is quite big, I have to imagine it has broken through the rock.