Well first of all, look at the cables carrying the current, how thick do you think the traces will be?
The current thickest trace for PCI-E is
"PCI Express cards are allowed a maximum power consumption of 25W (×1: 10W for power-up). Low profile cards are limited to 10W (×16 to 25W). PCI Express Graphics 1.0 (PEG) cards may increase power (from slot) to 75W after configuration (3.3V/3A + 12V/5.5A).[9] PCI Express 2.1 increased the power output from an x16 slot to 150W so that some high-performance graphics cards can be run from the slot power alone.[10] Optional connectors add 75W (6-pin) or 150W (8-pin) power for up to 300W total."
1) It's a pain in the arse to push the cable onto the motherboard (Very recessed)
I would certainly keep the connectors there at the edge and the most efficient method with a vapor chamber is with a onboard fan ( I'll tell ya, doing what you mentioned will require a massive heatsink )
Look carefully, they will randomly place connectors all over
That last graphic card chews at least 400W