Is there a way from this data to calculate the frequency of the wave(s)? Bandwidth? Or otherwise characterize the signal that is causing the polarization? Is that even a meaningful question in this case?
Not really. What's being observed are remnant polarizations from gravitational waves that had their effect long ago, under very different circumstances and mass-energy densities.
What's interesting is that the present measurements can be interpreted as evidence for gravitational waves to the exclusion of other explanations to a high degree of certainty.
Until now, evidence for gravitational waves was rather indirect and circumstantial, for example orbiting pulsars (very dense collapsed stars that emit periodic radio pulses) were observed to slow their pulse repetition rate over time in a way that suggested they were losing potential energy by radiating gravitational waves. Unfortunately those waves could not be detected directly.
In principle, a gravity wave could have nearly any frequency/wavelength consistent with its source. The pulsars discussed above were thought to produce gravitational waves of relatively high frequency / short wavelength, proportional to their pulse repetition rates. A so-called "millisecond pulsar" would have a possible gravitational wave frequency of one kilohertz and a wavelength of 3 x 10^8 / f meters or 3,000,000 meters (3,000 kilometers). That's hardly short compared to a radio broadcasting station's wavelength, but for gravitational waves, it's remarkable.
