Correct, you don't want to push voltage to the mb v out, prob wouldn't end well.
Are you saying that pushing 12v power down into the motherboard from the pci-e slot is somehow bad? I don't think it works that way. If you think it would fry your computer, then why would a product like this be sold from a big name GPU brand?
http://www.evga.com/Products/Product.aspx?pn=100-MB-PB01-BRIMO, I would prefer to buy something like that than mess with 6 individual risers and 6 individual molex. Powered risers are just a mcguyver solution. Having on-board supplemental GPU power is the best solution (MSI Big Bang XPOWER II has a PCI-E plug on the motherboard to supply dedicated 12v power to PCI-E slots, and some higher end Gigabyte boards have a SATA plug to achieve the same thing). Getting the EVGA Power Boost is a solution between the mcguyver and the built-in. It's professional, and neat/tidy.
edit: hashratestore's risers feeds 12v power back into the board, so other cards can draw off the molex too. Only 1 or 2 of his risers should sufficiently prevent ATX plug burnouts when a full complement of GPU's populates a motherboard.