If you put a MOSFET in backwards does the impedance change?
Consider the case of a diode. Diodes have a low "resistive" impedance (i.e., current is in phase with voltage), and they also have capacitance. At low frequencies most of the current flows through the diode resistively, and at high frequencies most current flows through the capacitance.

When the S-D of a MOSFET is biased backwards, it's exactly the same thing. It's a diode.

Be mindful that under forward bias, a diode has more capacitance than under reverse bias. Namely, the capacitance (called a "diffusion capacitance") scales linearly with the DC current. Therefore, the frequency at which capacitive current dominates resistive current remains roughly constant regardless of DC current level.

To answer your question of whether you can ignore the source-drain capacitance: It depends on the frequency of interest. 

In the synchronous FET of a switching converter (which is where you'll usually see a FET with reverse S-D bias), you do care about this capacitance. In this context though, because the behavior is large-signal and the frequencies are so high to cause non-quasistatic behavior, it's usually called something different: reverse recovery charge. This emphasizes that in this context, thinking of the diode as a simple conductance or capacitance is incomplete.