Why are replies to interesting/fair questions so often something like: "this isn't important or a good idea"
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How hard is a fair question... it's possible, but require a 2nd render of the scene for each visible side. Not sure how hard, but a few hours of work (best case)... and the performance would suffer. Probably more like a day or two.
I agree it would be cool, but we probably aren't going to get around to that any time soon.