Hi Ben,
You didn't specify your Vin and Vout specs for your application. We used Webench on the LM22678 for 12V input and 5 V output the the default simulation mentions 20mv p-p. We will use this as the starting point for discussion. In practice, the ripple will contain high frequency spikes that extend well past a 100Mhz. These high frequency spikes can be suppressed with low value ceramic bypass capacitors. Most datasheet specs for switch mode power supplies ignore the high frequency spikes and only quote p-p ripple at the switching frequency. You are also correct in observing very few datasheets for SMPS IC's, give PSRR in the same manner as is done with linear regulators. The reason is that it is difficult to properly inject the input side AC signal and get a proper measurement. In contrast this is very easy to do with Linear parts.
Input noise that is within the bandwidth of the switcher (up to 1/2 the switching frequency) can reappear on the output of the switcher. The degree of which this is a present depends on the control topology of the IC.
Sincerely,
Anston