I think most Denon and Marantz AVR's still have RS232 ports and you'll find one on most older mid-to-top end AVRs on the used market.
If you don't need to read data from the AVR and just need to control it then you could use a universal remote that itself has an API or use an Arduino with an IR diode to send it the the relevant codes.
You don't mention what you're using to control your home automation, but it might be that a physical serial connection isn't the best route anyway, some of the newer AVR have built in APIs that allow you to control them over a local network.