How linear does it need to be?

I think you could use an XOR gate and lowpass filter the output to get an average voltage.

The two inputs of the XOR are 1) your clock signal and 2) a delayed version of your clock signal.

This delayed version would have a delay proportional to the frequency. To do this, I would stick the clock signal through a lowpass or a highpass filter. This will provide a phase shift proportional to the frequency. Note, I am assuming the voltages are large enough such that the XOR can still process the delayed version correctly.