[whpalmer4]

I specialize in answers involving things you can do with the current product, not what you might be able to do when some other feature arrives in the distant future :-)

You could directly read from the (cached) database using sqlite3 if you know SQL and can puzzle out the encoding of the various values in the tables. I don't know if it is easy to rummage around with Xquery in OmniFocus databases. One potential sticking point is that your database is rarely contained in a single XML file.

As Curt says, if you want a date completed column (or anything else), ask for it with Help->Send Feedback to get your request officially noted.