Thanks for those suggestions. I did not know about the "dont detect serial ports" option. Good info.
However...
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.