Could I create an energy sink (huge space heater?), that only turns on right when the grid is about to fail, and then get paid not to run it?
IIRC that'd equal vaporizing water at 125 kg/s, doable but that's 10,800 m³ water per day.
Aaaand, in order to get the kind of contract from a power utility where they pay you to turn off, you'd probably need to operate a decent fraction of the day, say 25 %.
So you'd first need to build this thing and then you'd need to draw 1.5 GWh of electricity per day.
At 50 USD/MWh, you'd pay USD 27 million a year for the electricity to operate.
Not sure this'll work out financially.
It is likely that it would be cost prohibitive to hookup that much load with that expected profile.