Technically possible, irresponsible, bad advice, and probably won’t work in most scenarios.
First the smaller circuits will trip first, that's where the current is actually flowing, there wouldn't be any actually flowing through the whole house breaker in your idea. The main circuit to your house is 100+ Amps (this would be a tiny old circuit most houses have larger main feeds these days) at least. You'd have to feed at that much through the main disconnect for it to trip.
Second even if you could push that many amps through the main breaker somehow to trip it you'd be feeding it through wires designed for 15-20 amp nominal loads which would cook them.
Well, you'd end up tripping the 15-20 amp breaker protecting those wires first.
The fault current that a breaker is required to be able to interrupt is two orders of magnitude larger than we're talking about feeding into the wire, and a 15 amp breaker is designed to _not_ interrupt a 150 amp load until a second has passed (there's a rating curve for this): some loads might require that much current briefly, and the wire in the wall is not going to overheat in that time (that's why the breaker has the curve that it does: to model and therefore protect the allowable heating of the wiring).
Doubly so because it doesn't even do what we want it to which would be to switch the main breaker off.
In addition to that you're feeding energy back in through a much smaller wire so you're limited to the capability of the circuit it's connected to and the devices have to be smart enough to actually limit their output to what the wall can handle. Otherwise imagine you have 2 of these each on 1000W circuits (not a normal circuit but for convenience lets pretend) and you have 1500W of load running elsewhere in the house. One runs out of power because it had a lower charge state when power failed, now unless the remaining UPS limits it's output and browns out everything you're pulling 150% the rated power of your circuit, potentially damaging the wires or the loads you're powering.
This is potentially what's causing fires on nVidia's high power connectors, one connection wears out or is simply loose so more power flows through the remaining pins and it's too much for those wires causing fires/melting the wires.