It looks like this capability is already installed in RM. In any rule info page a lot of installed capabilities are liusted:
installedCapabs [ColorMode, Polling, PowerMeter, PushableButton, IlluminanceMeasurement, CarbonMonoxideDetector, Battery, Outlet, DoorControl, MotionSensor, Alarm, SpeechSynthesis, ColorTemperature, PresenceSensor, SmokeDetector, Thermostat, Light, AccelerationSensor, Refresh, RelativeHumidityMeasurement, TamperAlert, EnergyMeter, AudioNotification, MusicPlayer, UltravioletIndex, TemperatureMeasurement, Momentary, SoundSensor, Lock, Notification, CarbonDioxideMeasurement, GarageDoorControl, SwitchLevel, WaterSensor, SoundPressureLevel, Switch, HoldableButton, Configuration, ChangeLevel, Actuator, Sensor, ContactSensor, LockCodes, ColorControl, PowerSource]
So, if I understand it correctly, it looks like this capability along with other missing capabilities are simply not present in RM GUI selection. Is it complicated to add?