I would simply use
Rogue Amoeba Soundsource and insert the AU plugin for that app only, and tell Soundsource to route it to the DAC. Soundsource is a neat menu bar item, and was made for things like this. It's $49, but I'm an old studio guy, my mac is very much built for audio, and I think it's worth every penny. However, I'd be surpriced if there wasn't a way to work around getting paid software to do this. Let's hope someone has a clever tip.