No putting any kiboshes on anything this time - sorry 
Unfortunately, it is as @dave_dore indicated: loopback ports only exist for INPUT ports. So yes, you could use loopback ports to communicate between songs and the master rack, but you’d have to use the loopback ports in your songs:
- create a new audio input port “MasterRack”, not connected to any physical port
- send the output in your songs to “Loopback - MasterRack”
- then use the “MasterRack” input port in your background rack to process the output from your song (compression, master EQ, limiting) and then send it to the physical outputs in your master rack.
- the advantage of this: you can use background rack states to switch between different audio configurations (mono / stereo, individual outputs vs. summed) and processing settings without having to do anything within your songs.
But as @dave_dore stated, you’d have to edit the routes in all your song files to point to the loopback port instead of the “Main Speakers” output.
On first glance, there should be no need to use complicated regex magic; it should be feasible to simply do a mass replace of “targetPort”: “Main Speakers” to “targetPort”: “Loopback - MasterRack” with a batch text file editor like AFR (mentioned by @dsteinschneider).
After this operation, your songs would then send all audio output to the new loopback port, making it available for your background rack to process and send to the physical output.
I’d definitely recommend backing up all your song files before doing that and testing it first with a few of your songs - not sure if this simple replace operation might not mess up some things outside routes. I’ve checked it in some of my songs, and haven’t found anything surprising, but you never know…
BTW: I’m just working on a simplification of my song files using loopback ports - I’ve moved most of my “Faders” rack connections to my background rack, which removes a lot of the tedious “standard cabling” from my songs. The idea to move the master rack to the background rack is a nice one - I’ll consider this as a next simplification step.
Currently, all my songs have this master rack as their final stage, but it would be a lot cleaner to move it to the background rack. Since all my songs already contain a master rack, making the switch will be pretty easy: create the master rack in the background rack, then change the linked master rack contained in my songs so that it simply forwards the audio streams to the loopback ports. Then I can change the songs one by one and replace the linked master rack with direct routes to the loopback ports.
Cheers,
Torsten