there are probably a number of ways to do it.
1) receive the signal, amplify it, clip it to be a stable power level, then use a fixed attenuator to drop the output level to 0 dBm. if the clipping causes some issues, lowpass filter the harmonics that are generated by it.
2) injection lock an oscillator to the signal. the oscillator is at a constant output power, so it can be attenuated to 0 dBm output. the only drawback, the oscillator runs even if there is not input signal.
3) a variation of 2) is the SuperRegenerative amplifier (a technique used in receiver back in the day). in that case there is not output if there is no input
4) amplify, send thru a variable gain amplifier that is controlled by an envelope detector at the output, then attenuate the whole thing to be 0 dBm output
5) amplify, sample the whole thing with an analog to digital converter that has a LOT of bits (high dynamic range). use digital math to change the digital gain, then drive a digital to analog converter.
6) have a VCO running at approximately the input frequency. Use an analog phase detector to phase lock the VCO to the incoming signal. Since the VCO output is constant in power, just attenuate the output to 0 dBm. use the large dynamic capture range of the PLL to catch anywhere from tiny to large input signals. You might need to limit the tuning range of the VCO to always be near where the input frequency will be.
7) set up an amplifier, and a delay line loop, to make a "frequency memory amplifier". i.e. you couple the input signal into the delay line, the amplifier boosts it, then sends it back to itself. you sample off the frequency circulating in the delay line loop for your output
8) amplify the input, send it AC coupled to a CMOS gate. Have the DC bias on the gate so it is right at the trigger level between a "1" and a "0". when the input signal is present, the gate puts out a square wave of the same exact frequency of the input signal, at a constant output voltage. then attenuate and lowpass filter it, and you have a constant power copy of the varying power input signal.
Come on guys, who can think of some more ways...