Finding it hard to understand your request. Do you want to measure something? You say
"measure the impact of short, sudden sags" - what impact? on what?
Surely you just want to measure (rms?) mains voltage, which is done via a transformer
and signal conditioning circuitry normally. Transformer brings voltages down to safe
values, an opamps / whatever to present to the ADCs.
Presumably, a power supply should be able to tolerate a lower input voltage on a few ms basis than on a continuous basis.
Mains voltage does zero-crossings all the time - power supplies cope with those events
all the time and they are "a few ms". Or do you mean more than a few?