What level of a square wave signal should be safe for TV receivers?
100dBuV is about the most signal you want to see on an antenna cable.

That comes out as 100mV RMS

This is not because the units are interchangeable. they're not. it's a coincidence.

so 141mV peak should not harm any equipment.

DiSEqC uses 22kHz (sine) at up-to 900mV peak-to-peak, but unlike your square-wave there's no RF component to that signal, so the receiver circuitry won't be effected.