Thanks for the swift reply, PMM! It seemed reasonable to me that I should be able to, but I was unsure because in the extension application I had to state my last date/place of entry to Canada – which would clearly change if I travelled in the interim - and the CIC website wasn't clear (to me, at least!) Specifically:
May I leave Canada before my request for a renewed or initial work permit has been finalized?
Yes. However, if you leave temporarily and your study or work permit has not been renewed before you seek to re-enter Canada and you do not have your new document, you will have to re-apply (either at the port of entry if you have the right to do so, or at a visa office outside Canada) and pay another processing fee.
Anyway, you've put my mind at rest. Kudos for that!