> So tossing in antimatter would not do much. It would never cross the horizon in an external observer’s reference frame and thus would never interact with anyt

> So tossing in antimatter would not do much. It would never cross the horizon in an external observer’s reference frame and thus would never interact with anything and thus would produce no effects. It would never be directly seen crossing the horizon by the accelerated observers you are thinking about (it just gets redder and dimmer), but it sure crosses the horizon, and this can be seen by those observers indirectly: the mass term of the BH metric describing the exterior spacetime changes, with observables like the Einstein lensing subtending a greater angle and a change in the apparent location of the ISCO. Consider the act of throwing a smaller black hole of mass M_small into a larger one of mass M_big: General Relativity tells us that within about a light-crossing time of the sum of the diameters of the two BHs there will be a larger BH of M_merged \lt (M_big + M_small) with the missing mass dumped into a large dynamical perturbation of your choice of background metric. (Linearizing this picture is literally how gravitational waves are studied; with the present detections of gravitational waves, we can be pretty confident that neutron stars do in fact cross into the horizons of black holes rather than gathering up as some structure microscopically close to the horizon, and even more clearly not as a long-lived bump near the horizon of a BH where J \gt 0). There are of course other observers who could outright see the infallers cross the point of no return. (Power-orbit yourself close to the horizon of an ultramassive black hole where the tidal forces at the horizon are even less than those we experience here on Earth. Poke your little finger through the horizon. By virtue of your orbit it will slice right off, and using a highly sensitive gravimeter you would detect it behind you as you continue on your orbit. If you don't believe in gravimeters that sensitive, feel free to substitute a prominent mountain on a rocky planet for your finger and you, and you could then use existing technology. (Also, you could use a highly elliptical orbit instead of a highly powered circular one) In either case, a distant outside accelerated observer has to contend with the redshift, and so won't directly see what happened, and might not be patient enough for you to communicate your results. Additionally, the "bump" will only survive for a fraction of your orbit around the BH, since the bump is hair that will be balded within about a light-crossing time. The ringdown will propagate as gravitational radiation which will get to distant outside accelerated observer sooner than you, or a message from you, will.)