Actually, the "house ground" is only used because it's convenient. You can also tap off the command base and just run a long wire between the tracks of the carpet the layout to serve the purpose of the earth ground. You can connect to either pin 5 of the serial connector or the outside barrel of the power supply connection.
Probably the easiest way to connect to the earth ground might be to use a 3-wire to 2-wire adapter and connect to the ground connection on the adapter.