How do I make an old Airport Express be a purely Airplay device?
The reset button can be pressed to temporarily reset the password of the device to the default. At that point, you would log in and put the device in bridge mode, update the password, turn of WiFi if you wish.

I've used these devices as you mention many times and it works well. The only catch is sometimes the latest AirPort utility won't manage older devices and you need an older configuration tool.