Writing workflows for the PDF button is relatively straightforward if and only if there's an Automator plug-in that does what you need...but at the end of this workflow, you need to clip the file into OmniFocus, and I don't think there's anything off the shelf that will do that. I understand that you can roll your own Automator actions with Applescript and Xcode, however.
http://developer.apple.com/mac/libra...roduction.html
Personally, I think I'll be able to print quite a few PDF files to the desktop and clip them from the Finder in the time that getting the fully automated setup working might take :-)