I have an older Denon AV receiver that has RS-232 but no Ethernet or WiFi. From looking at the Denon documentation, the RS-232 control commands are exactly the same as the commands used over the network, just a matter of a serial interface instead of a TCP socket.
What I am wondering is if I could use an ESP device as a terminal server (convert the serial port to a TCP socket), and then use the Home Assistant Denon integration with that. I assume it would work the same way, open socket, send command, just not sure if the integration would allow for that.