Have you tried using the SNMP Cisco System Health Sensor?
While deploying the sensor several variants will be offered, including the "Cisco System Health Other". This variation of the sensor will display TX/RX power readings (if available/reported).
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.