Schneider EGX100 Gateway with AB PLC 5 series
OK. I figured it out. The steps to do this are as follows:


Setup Modbus TCP/IP filtering on the EGX100 through the webserver access to enable the Master device (PLC or PC) to have read write access. To do this you need the IP of the master.
Setup the appropriate read / write packets from the master giving careful consideration to the Unit ID parameter. This will define which slave the command is sent to.
Automate :-)


I think its also worth noting that the Schneider provides a Web Page Generator utility that can be used to define slave device specific web pages on the EGX100 (and enable the 'Monitoring' tab). This maps the appropriate registers to the actual metering values so for instance, instead of registers 4000 equals 600 you get Power Active = 600.