Thanks for those suggestions. I did not know about the "dont detect serial ports" option. Good info.
The only way I've been able to send commands to the receiver is to install with the script. I have just enough time (before the next PNP script activation) to open webadmin, select the device, and send a command or two. Oddly, even though the receiver works, the template says "please select" in the comm port box (in both the AV and GSD device. When I select ttyS0 and update the template, then I find that the GSD template has been automatically updated with ttyS0. But after issuing a reload router, the Yamaha no longer responds to commands.
Next I tried the "don't detect serial port" option. I added the GSD manually, but again the Yamaha does not respond.