Measuring impedance frequency response on Low Power Board
Correct me if I'm wrong, I think what you are trying to do is verify that you have appropriate bypassing and that the layout is not susceptible to some coupling. 

Getting the impedance frequency response sounds trickier unless you already have equipment to do that. What I would do is first check the frequency response of the bypass caps. You might be able to get away with using a signal generator as a power source. Put in the required DC offset and add an additional sine wave. Ensure that the peaks / lows are within spec and don't trip a UVLO or something like that. Get a nice probe &amp; scope and stick it on the node of interest (MCU VDD for example). Measure the amplitude of the signal at the node and you can calculate how much attenuation is occurring. From this you can calculate your power supply rejection ripple (PSRR) or whatever value you want.

Alternatively, inject some white noise into the system, and grab a spectrometer of the node of interest. Once again, you'll be able to see how the board is rejecting unwanted signals.