Phenegma & Community,
It turns out that my receiver is an AVR-2307, which does not support Ethernet or CEC. The only option is RS-232. I have connected the receiver to my hybrid using a PCI serial port card with two ports and a db-9 cable. I do not know the pinout of the cable though there are male and female connectors, matching the genders on the equipment.
The wiki page, which is old, describes that this is plug&play. Trying the setup wizard, it is not detected on either port.
Trying to add the receiver manually, I selected Wizard > AV Equipment > Add Device and picked the AVR/AVC (Ethernet) template. It shows two serial ports, ttyS4 and ttyS5. I am ASSUMING that these correspond to the two on my PCI card. I tried both but still no luck after running AVWizard again to assign the inputs. Sending the "off" command through the Admin tool also does not work.
My normal troubleshooting approach with serial devices is to set up a dumb terminal on one side of the connection, send commands, and see the responses. This is the path I'll go down barring any advice. For starters it would be good to know if the use null modem connectors is typically required. Also, are there linux config files to validate the /dev/tty entries associated with my PCI card?