Finally I just checked and I can confirm you can route the master to the A and B jacks on the SQ rear panel.
To do this :
I/O page > “Mix Out” page > Select “Local” > scroll the page to the right (you can see A and B) > scroll page to down until you can see “Main *” > check the corresponding case ;).
Default setting is : Mtx 1 L and R to 13+14 and A+B Outputs.
If you want to control the master and monitors volume separately best way is pass by matrix. Route your master in 1 of 3 Matrix and route Matrix to Outputs of your choice.
Cheers