There is a Output Balance control available in the Main Send settings. You can find this by pressing the Sel key for the Main bus, then press the Routing key and Main Send Settings button on screen.
The output balance control for a Mix Bus is a socket balance which is part of the routing of the bus. Where as a group has a socket balance (part of the routing) and a bus balance (as it can be routed to LR).
It is therefore not possible to have both controls on one encoder.
This is also something that you wouldn’t normally change mid-show, so not being present on the surface avoids this.