Thanks Reburns, I have also posted this question on the Bike Mechanics forum and it has been pointed out to me that a press-fit hub (as this very clearly seems to be as the axle is not threaded on either end and the hole is not a hex socket, but rather is for the quick release skewer) is not suited to my original plan of moving a single drag brake from one tandem to the other as the friction fit would deteriorate with each time it is open and closed.

I am now considering two options - either (1) obtain another drag brake or (2) a different tandem hub that does not use press-fit spacers. All of it makes me wonder about my plan (that was honestly started because the bike coop where I volunteer had this 26" tandem wheel that it wanted gone when it was temporarily shrinking due a move). Eventually, I hope my son will grow and we will ride the larger Kuwahara tandem with the 27" wheels (which deserves a post of its own for me to sing its praises).