Install OS X 10.6 on the Retina MBP

Basically impossible. Since the hardware support is baked into the OS, and not really released separately like with Windows, you're basically SOL.

You could try grabbing the newer kext's (equivalent of Windows drivers) and getting them to work with 10.6, but honestly the only thing I would expect that to get you is a big fat kernel panic.

You might also try looking to the Hackintosh community, who are quite adept at coaxing Mac OS X to run on "odd" hardware. It's possible someone has (inadvertently) solved this problem.

Both of those ideas are long shots, but that is basically all you have.

The older version of Mac OS might have trouble understanding the extremely high resolution display. Apple put a lot of work into getting that display to look sharp without requiring a microscope to be seen but if SL takes the resolution at face value you'll end up with everything looking tiny on the screen.