On my installation of Entourage 2008, I have a vast hierarchy of files with names like:
/Users/paco/Library/Caches/Metadata/Microsoft/Entourage/2008/Cigital/Messages/0T/0B/0M/6K/6224.vRge08Message

These files, if you open them using the "open" command on the command line or double click on them, cause Entourage to fire up looking at a specific message.

The files actually seem to be an XML-ish plist-ish representation of the actual email messages. Thus, Spotlight can index these. If you used mdfind to find one of these files, you could link to it. Then double clicking on that file name ought to have the desired effect.

I'm too new to OmniFocus. I haven't started integrating it into all my tools, yet. When I saw this thread I thought I'd post what little bit I knew.

Paco