Using the RSSI as a rough measurement of signal power is a common idea. We would not recommend the DFO however (that signal is still being used to decode the baseband)—rather, we would recommend using the PDOUT (Peak Detector Output). The PDOUT signal is a little more consistent and is isolated from the actual data fluctuations in the baseband signal chain.
Unfortunately, the MAX1473 is not designed to be a RF signal power detector. The linearity is specified at ±0.5dB but as noted, the offset is not—it is not designed to be absolutely consistent.
There is an additional issue with using the RSSI as an absolute power detector: the Automatic Gain Control:
That discontinuity in the curve can make the absolute signal strength difficult to determine. The AGC can be shut off with the AGCDIS pin however, the LNA will saturate at a much lower power level.