The spacers pass right through the mandrel with the nuts left in place Howard, and a rotating plate with three keyhole slots is used to retain the chuck. Without those caphead screws getting in the way the backplate is hard against the mandrel with a register protruding into the central hole. It is much more convenient than the more usual Chinese flange and nuts, taking well under a minute to swap chucks.
The 3-jaw uses hex head screws that have plenty of room in the recesses, and I used the same both on the self-centering chuck (with a modified Sieg SC6 backplate), and to replace these cap heads. As you said, they aren't going to be removed very often, and there is plenty of room to use a 13mm open end spanner.
Brian