I just made this yesterday for the same thing! It's not perfect because you have to always start from an L-R starting point, but you can create a KS to open the Modify Audio Channels dialogue in Premiere, then you have to do a series of KS "TAB" until the necessary box is highlighted, then KS "Space Bar" to select or deselect – then KS Enter.
Again, it's not super perfect and it was a little tedious to set up for both RR and LL. I also made one for cycling through Audio Channels that come in as 1,2,3,4 (boom, lav, etc).