Ok, so it was as I thought - it needs to be done by the output plug-in due to output device selection being specific to the output mode used. Not much I can really do with the DirectSound plug-in as I don't think there's an interface it exposes to live change things either (you'd be looking at best a hack that causes a restart to allow for the config used to be manipulated which defaults the point of what you're wanting).
-dro