Here's my Hazel rule:
![Image](http://www.hamishmacdonald.com/uploads/HazelDMGprefs.png)
And here's the Automator workflow:
![Image](http://www.hamishmacdonald.com/uploads/DMGer.png)
(I know there are similar processes here on the boards — which led me to this — but none of them did quite what I wanted. I hope this'll be helpful for anyone else who finds DMG-packaged installers a nuisance.)