For others who might find this, I ended up using ImageMagick for the jpeg thumbs with this:

find . -regextype sed -regex ".*/[a-z0-9]*\.jpg" -exec convert {} -thumbnail 266x200 {}_thumb.jpg \;

within the adapt_authoring/data/master/assets/ folder.

A bit more tedious was editing each database record in the assets Collection (using Robomongo as I'm not yet comfortable working with mongo on the commandline).