Is there a way to install/hack iMessage onto Lion/10.7?
Since the Messages beta has ended, it is no longer offered for download on the website however the download link direct to the .dmg still works:

Messages beta direct download.

I don't expect this to continue indefinitely.

A more permanent option would be to try and copy the Messages.app from a Mountain Lion install on another computer to your OS X Lion applications folder. Let us know if that works for you.