Physical idealism

[Neo-Scholastic]

I'm not sure how you could derive the supernatural directly from Platonism unless you simply equate nature with the physical world...

That was pretty much what I was doing. To me the natural world is defined by all that which is governed by physical interactions. This definition seems to be consistent with what most atheist members of AF believe. That would make any aspect of reality outside of the physical, like geometric figures as-such, supernatural.

There's nothing intrinsic to the idea of immaterial beings (such as geometric figures) that must be excluded in one's rejection of entities represented by the admixture of attributes oftentimes annexed to the various gods.

On first blush that would appear to be the case. The Divine Mind enters the picture if one believes, like I do, that neither the fully transcendent Ideas of Plato or the fully manifest forms of Aristotle addressed the problem of Universals in a satisfactory way.

Plotinus, if I recall, also believed that neither existence nor non-existence, being nor non-being, could be predicated upon "the One" or the "All", as it was wholly incomprehensible.

Yes. It would be fair to call Plotinus a mystic in that sense.