I have Landis and Gyr meters behind a firewall at a client's factory on a LAN, communicating via TCP/IP plug in modules. I need them to connect to a Plug and Play Scada server on the Internet. Can I set up the Landis and Gyr internal TCP/IP modules to connect out to Plug and Play Scada on the Internet, through the firewall?

Yes.
The problem is that the Plug and Play Scada server on the internet cannot see the etherpads on the meters because they are behind a firewall. The solution is therefore for the etherpads to connect out to the server's internet IP address. When you do this, let each etherpad connect to a unique TCP Port number on the Plug and Play Scada server. You can do this by adding an Active Etherpad for each meter, and specifying the unique port number when you add the Active Etherpad. Plug and Play Scada will force you to specify a unique port number for the Active Etherpad, and you can rest assured that no-one else would have allocated that same port number if you have successfully added the Active Etherpad with that specific port number. So it generally make sense to add the Etherpads on Plug and Play Scada first before you configure it in your meter comms module.
The way you set up the Landis and Gyr comms mudule, is by setting its IP to be dynamic, using the meter programming software (MAP110 or MAP120). This said, you need to have a DHCP server on your network that will assign the necessary TCP/IP settings to each meter dynamically when it starts up. The unique MAC address can be retrieved by using MAP110/MAP120: use this to assign any unique IPs to your meters within your DHCP server. Remember, you must also give each meter a gateway address so that it can break out to the Internet (and set up your firewall accordingly).
Once you have done this, another field will become available, called the IP Address Central Station. This must be the IP of your Plug and Play Scada server. To get the IP of your Plug and Play Scada server, ping the domain you use in your Web Browser to browse your server, e.g. sdg.pnpscada.com is at the time of writing equal to 196.22.139.148. Please note that the TCP Port setting is the TCP port on the server to which the etherpad will connect, if the dynamic IP option has been specified. In other words, it is the port you specified when adding the Active Etherpad in Plug and Play Scada. (If the static IP option is specified, the etherpad will not connect out, and the TCP port setting becomes the local TCP port to which one connects on the meter itself. So the same field is used for two different things, depending on whether the IP is static or automatic.)