Looking at your pic, it seems clear that the vent holes are not at the highest part of the case, when the Mac is mounted at that angle. Would propping it forward such that the vent is at the top help?

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True, but the highest part of the case contains only the top of the CRT. As I found when designing the Fanny Mac, all you have to do is actively suck the hot stagnant air out of the case, and the works will run much cooler. Obviously from a cooling perspective it would help if the air outlet was at the very top. An iChimney would be cool!

I am not a fan of pure convection cooling because until you have hot components there's no cooling. I don't like hot-running stuff, therefore a fan is required, though it doesn't usually have to be as intrusive as a Wind-Tunnel G4. A small 12V fan running on 5V or 7V is near silent and is sufficient for cubes, many video cards, and iMacs.