Late-2011 MacBook Pro’s and Snow Leopard
I personally haven’t done this but from research it is possible:
This article helped 

Method 1: If you have no cooked backup of your drive
This method will wipe your Mac but you can do a backup:

You should backup the locations where your documents, music e.t.c are stored to an external drive

Next insert the snow Leopard disk, restart your Mac and hold down c too boot to your dvd drive

Once the installer appears, go to utilities in the menu bar and click disk Utility

Find the drive in the sidebar where Lion is installed, select it and navigate to the Partition tab.

Click on the drop down menu under Partition Layout and select “1 Partition.” Hit Apply.

Now navigate to the Erase tab and make sure “Mac OS Extended (Journaled)” is selected, then hit Erase to delete all data on the
drive.

Quit Disk Utility to get back to the Snow Leopard installer and proceed with the install.

Restore your User folder and documents from your backup.

Run Software Update to get  everything current, install your apps  from your original install media and  update those as well.


Method 2: If you managed to clone  your drive
A popular option for cloning your drive is Carbon Copy Cloner, which
is a handy utility to have for any Mac owner. Assuming you’ve done
that, follow the directions above to get your drive primed and ready,
but stop at step 6 and follow these instructions instead.

Attach the drive containing your  CCC clone to your Mac.

Open System Preferences, then  go to the Startup Disk preference  pane.

Chose your CCC external drive  as the startup disk.

Restart and launch CCC, then  choose your backup from the  “source” menu.

Select your Mac’s internal drive  as the destination.

Chose “Restore items from a  backup” from the settings pop-up  menu and hit the “Clone” button.