I checked (which I know you already have) the Booth Compendium, Footnote.com, the National Park Service site, Rootsweb and googled Griffin, but could find no information on Owen David Griffin in the 5th or 6th LA Cavalry.

The only other thought that comes to mind are the files of the Jackson Barracks Military Library (JBML) in New Orleans, but unfortunately I think it is still closed due to the impact of hurricane Katrina in 2005. I have tried to e-mail JBML and have put a question on this message board to see if anyone has more information.

Sorry i couldn't help more

Don Parker