HI Ian , Still not totally sure whether you can or can't. By the looks of the plugin you should be able to address multiple slaves. BUT that may be going through another Modbus unit and then out to more Units indirectly.
I have read where it may be possible thru the Brains side , But brains can be very limiting themselves.
Probably the person to ask would be Peter Homann as he is the supporter of Modbus in the Mach3 world.
I'll give him a shout to make absolutly sure(maybe) , (;-) TP