The communication from the PLC needs to be converted to the OPC standard, either via OPC-DA or OPC-UA. OPC-DA servers need to be ran on Windows, because it is COM based. I doubt you will see a native UNIX OPC-UA server this early as well. Looks like your sticking with running some box on Windows my friend. Also To get FactoryPMI to talk to a PLC, you need some sort of transaction manager, in this case, FactorySQL, which only runs in Windows.