After a call to Topeak's service company, Judson, they confirmed that the thin edged cap just above the hose hook, will indeed unscrew. If you don't have a pair of pliers with you, the locking protrusions on the base of the handle can be finessed to do the job.

They need a minor redesign of this cap, thicker with a serrated edge, to make it easier to loosen. Or redesign the base of the handle so it can be used to unscrew the cap.