The circuit-design indicate that the main XLR and 1/4" outputs are physically connected, but I wonder how much separation there is between the left and right side as there are option to make the signals mono or stereo. The physical layout looks like left and right are separate. If those are separate signal paths then it should be possible to re-route the monitor out signal to use one of the XLRs as monitor out. I.e an option to make the monitor-out volume, EQ and monitor-out-link apply to one of the main outputs when it is set to master-mono. Maybe make it another option in the source selection for the main output called master-mono+monitor that makes the left-output a normal mono out and right-main a monitor out,
For future hardware-designs I would strongly suggest combined stereo balanced/unbalanced outputs for monitor just like the main outputs. Or even better: create a multi-output matrix of balanced/unbalanced outputs that can be freely assigned to different roles as single/mono or stereo-pairs.