After upgrading Snow Leopard to Lion, can I reinstall the original Snow Leopard to a virtual machine?
Yes - it is technically possible. 

The Leopard and Snow Leopard installers look for a file (even if it's empty) named /System/Library/CoreServices/ServerVersion.plist

Armed with this, you should be able to search for many "how-to" articles like this.

Be aware, the comments on the page call out issues that this "hack" causes when running the OS (software updates is confused by this file's presence/absence) 

Legal and moral issues aside. (I'm not a lawyer but I too fail to see how the license prevents you from virtualizing your legally purchased Snow Leopard on a legally purchased Mid 2011 MacBook Air)

Think of this as an iOS jailbreak - someone has to stay on top of the things that these changes break, but the payoff may be well worth the hassle.