Hello,
I think I have a working solution in applescript!
Essentially the script computes a link to the current OmniFocus project/folder and opens an Evernote window searching for anything that has a source url that matches that.
The script also checks before displaying the search window that Evernote would return at least one note - if it wasn't going to do so it automatically creates a blank note in Evernote with a useful title and the source URL set up.
Obviously from within evernote you can click on the source URL to go back to OmniFocus.
See:
https://gist.github.com/814226