Geoff,
the supplementary switch is basically a double pole double throw switch pack broken down into two separate switches. There are six terminals total for the two switches. Each switch has three terminals, comprising a centre ‘common’ pole and a terminal either side of the common, depending on the selection of the switch motor.
One possibility is to use one of the switchpacks using the centre terminal as the power feed and the terminal to each side as the 'red' connection and the other as the 'green' connection on your signals. The other switch assy could be used to power frog polarity or a track isolation circuit to stop the train on a 'red' signal.
Using a multi-meter it is easy to check which terminals are 'active/conductive' as the switch machine is moved from side to side. As far as AC power being used to power the 'DC' switch machine, then the AC supply is 'rectified' to single wave DC in the LGB switch control box.