It turns out that Automator has an Aperture "Extract Metadata" action that has access to all of the EXIF fields.
I was able to solve my problem using a simple Automator workflow that performs the following steps:
- (Aperture) Get selected items
- (Aperture) Extract Metadata -- With the option for "Tabbed Text" output.
- (Text) New Text Document
I have uploaded the Automator workflow as Extract Focal Length.workflow for reference. This workflow took about 30-minutes to run on 10,000 images, but at least I got the data I needed out.
One note, be sure to collapse the steps so that the results aren't displayed in Automator if you are running this on many items as this will slow it down dramatically.