I found an article from someone who managed to hack-install this: Installing OS 10.6.8 on a Mac Mini built for 10.7 – yes, it’s possible.
It’s definitely a hack and involves putting one Mac (the newer one that shipped with 10.7) into target disk mode and using another Mac (that came with 10.6) to install Mac OS X onto it.
While it may work, this is definitely an unsupported configuration. I assume it will become less and less likely to work as the target Mac’s hardware gets newer and newer.
Using a VM is definitely the better approach.