Filter ripples and their effect on signal quality
can one calculate the RMS phase deviation (or group delay variation) knowing the ripple amplitude variation (expressed say in terms of peak to peak)


Generally not. There are many different systems that have the same amplitude response but different phase responses. If you know that your system is minimum phase, you can actually calculate the phase response from the amplitude response (using the Hilbert Transform) so this may help.

The amplitude variation alone is rarely helpful since they are many systems that have the same variation but very different absolute responses.