Can I tell what the Modbus Plus address of my Modicon PLC is via 984 ladder logic?



Yes, the contents of the address configured in node 2 of the MSTR Instruction Function Code #3 (Get Local Statistics) will give you the Modbus Plus Network address of the PLC.
Below is a example of a MSTR Instruction using Function Code #3 (Get Local Statistics) to get the Modbus Plus address of the PLC

400001 = 3 (Operation Type)
400003 = 1 (length)
400004 = 2 (Offset)
400010 = 2 (Network address of this station)