Not only would that require me to buy an AirPort Express per speaker, many Sonos speakers don’t have an audio in jack. Thus my desire to bridge this purely in software.
It’s a bit janky for what you want do to, but if you have any Sonos S1 device with an input, you should be able to group that device with whatever other speakers you want playing the audio input. (And if that input device is a speaker that you don’t want playing, you can turn the volume all the way down.)