The DFDR in the 777 appears to record 128 samples / word per second. The parameters of interest to fuel flow and engine performance are sampled once a second.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.
The maximum sampled frequency is correct, but whether you can record something of interest is not so clear. If the sampling isn't low-pass filtered, you will still record aliases of the higher frequency. Basically you record a frequency on the difference between the signal frequency and the nearest integer multiple of the sampled frequency. So if there was a resonance at 21.25Hz, you'd record an alias frequency of 0.25Hz (21Hz being the nearest multiple of the 1Hz sampling). You couldn't tell from the record what the original frequency was, but you would get a recording of something unusual, so long as the sensors really are capable of doing point-sampling rather than one-second averages.