If you had a mixer in the Global rackspace that sends the audio to the output interface, you could simply change the wiring of the L & R output to both feed into the Left output of the audio interface. You can use the “To Global Rackspace” and “From Global rackspace” plugins to route audio to and from the Global rackspace.
I set mine up like this and always send audio to the audio interface via the Global Rackspace. All my individual rackspaces send the audio out via the Global Rackspace.
Effectively you would change this:
to this:
which should only take you a few seconds to achieve each time you need to.