Mono output from an external DAC
I'm going to end up sounding like a one-man advertising campaign for this app [I have no affiliation, it's just darned useful]

Rogue Amoeba's Audio Hijack can do this type of routing easily. It unfortunately cannot route 'everything' to any given output in a single move, but you can make it route multiple apps &amp; System audio through the same signal path.

You build by dragging &amp; dropping elements from the right side-bar into the main window. Connections are built as you drag. Then you dial in your specifics by clicking at the bottom of each element &amp; choosing its app, effect or device.
Effects are - some built-in or any AU effect plugins on your Mac.

It is ostensibly an audio recorder, but if you don't put a recorder element in the path, then it will just pass-through instead.

I built this in less than a minute to route system &amp; 2 yet to be decided apps through a channel 'effect' set to mono, then out through my audio USB.

Caveats:
It must be set running to function, so it's not a complete 'set &amp; forget' at system level.
It's expensive for a one-trick pony.
It will auto-launch all apps used in a 'Session' so you might not want to put them all in one setup but make different combinations, as needed.



See Split Input Channels Using SoundFlower and AULab and Nice Audio Mixer/Repeater for some other routing examples I've built.