Ok so here's how too,
Create 2 Tracks, one will be a ghost track which you could name something like:
GrooveAgent 2
GrooveAgent 2 Ghost
On Track 1 click IO and untick Master/Parent Send, now click add new send and select 2: (track 2), this will setup a send on track 1 and a recieve on track 2.
Now click add new Hardware output and select your normal output you use for Reaper.
Now ARM (red R button) and Enable Record Monotoring (Speaker Icon Button next to the In Button) then make sure on the IN Button its set to Record: input.
Thats Track 1 finished.
To Track 2, Click the IO but this time leave Master/Parent Send ticked. You should see the Receive on the right of the IO window which was setup when you made the send on track 1.
Track 2 does not need to be Armed or Monitored.
Ok that's the setup is all done.
How to use:
Mute Ghost Channel to mute to ReaNINJAM.
Also lowering or muting ReaNINJAM Channel will help prevent your levels of your tracks increasing when un-muting the Ghost Tracks.
Maybe in the future we there could be a monitor button in REAPER that allows for routing to any audio out. So we can just click a Monitor Button and it routes to the output we setup, this will be great for mixing on the fly and add further ability to REAPER.
You could shrink the ghost track down as its only purpose is to use the Mute button on it.