Yes it is possible to adjust brake balance during race but only at ace level. GPxPatch's GPxLap has facility for this. Enable in car setup changes in GPxPatch's GPxLap tab. Use L key to toggle CAR setup menu from cockpit (as opposed to Shift+L which gives PIT setup menu). Use F1 key to navigate to brake balance and F3/F4 to adjust down/up. F3+F4 together resets back to the current setup value.
This only works at ace level because F1 (auto brakes) F3 (auto spin) and F4 (no damage) keys are available because those driver aids are disabled due to ace level. The reason those keys are used is because those keys are recorded by GP4 for replays and hotlaps. This way the setup adjustments are also applied during the replay/hotlap.