As you suggested, the power dissipation profile over frequency or time can be rather complicated and is difficult to model. Therefore, in an effort to get started the model for IC power pins is an ideal current source with a resistor and capacitor in parallel. The current source can be a triangular pulse, double triangle pulse, sine, guassian, or another that I forget at the moment. You can define the parameters of the current source, resistor and capacitor to best approximate the power draw of the device you are modeling.
You can try it for yourself free online.