If you take the SPICE model (or a real part) and sweep an AC current sink at the output you should be able to get a good idea of what the characteristics are over a range of frequencies.
At relatively high frequencies, the output capacitance (eg. 1uF ceramic plus any other bypass capacitors in your circuit) will dominate, and at DC, obviously, all the current comes from the source and the capacitors have no effect. At intermediate frequencies the output impedance of the regulator, input and output capacitances should have a noticeable effect if you model the circuit accurately (such as source impedance at the input).
I don't think you should be using dBm for PSRR - the specification is based on voltage ratios so 20 log(x/y) gives the ratio in dB.
It's not uncommon to have some kind of input filter on a power supply to control conducted noise. Typically you put an LISN (line impedance stabilization network) in the input circuit ahead of the filter to make measurements.
Edit: Here is a plot of a simple LDO ADM7172 with a 10mA peak load applied over 100mA DC base load.