Thanks Grant! In the Apple Store, cables page, (as you noted) I don't see any extension (i.e. male/female) cable for sale. But they do have the 2 m lightning-to-USB-C cable ($35), which is evidently a longer version of the 1 m cable that came in the box.
Alternately… If I end up buying a USB hub, with the idea of killing 2 birds (lengthening my keyboard's reach + giving me more connection options), do you have a recommendation for a suitable hub?
Obviously, my original question remains - - will the keyboard work if it's routed through a hub and back into the original jack on the back of the Mac Pro??