Right now, we are using a somewhat hacky way to inject RSSI packets into our serial stream. However, it looks like the RFD900+ can inject RSSI info if it detects that we are using the MAVLink protocol. If it detects MAVLink heartbeat packets, it will start injecting that RSSI information using MAVLink.

Also, it seems MAVLink could replace Protobuf for Ground Station communication. It has the similar idea of defining messages, along with a code generator. It can even be used for both TCP and Radio, making the codebase easier to deal with.

For more information:
https://ardupilot.org/copter/docs/common-3dr-radio-advanced-configuration-and-technical-information.html#mavlink-reporting
http://files.rfdesign.com.au/Files/documents/RFD900%20DataSheet.pdf