Can off-line SMPS magnetics be used for wideband isolation of power line waveform measurements?
As it's designed for high frequency switching, its primary inductance will be hopelessly low for 50Hz or 60Hz use. So it'll appear as a low impedance shunted across the AC supply or system under test, drawing a high current.

That current, in turn, will saturate the transformer core, further decreasing the inductance - unless the measurement is across a low voltage.

Hopefully, if you are measuring across the AC mains supply, it'll just trip a breaker or blow a fuse before anything worse happens.