I didn't look - apart from making it pretty, what did Apple do with video beyond a PCIe x16 riser (like http://linitx.com/product/pci-express-x16-gen2-flexi-riser-7...)?

They have some custom pcb motherboard in there, a simple mini-itx one would have to pass through that central column and the ram would be sticking out on the same side.