Thickbox already lets you navigation using the keyboard, but it uses the < and > keys, not the left and right arrow keys. Solution is explained here: http://www.prestashop.com/forums/topic/67325-solved-using-navigation-keys-in-thickbox/
http://codereview.musicbrainz.org/r/2282/
Apparently wasn't added to the fix version.
http://codereview.musicbrainz.org/r/2282/