Well; let's just say that WHATEVER you throw overboard generally aint coming back onboard except for maybe the victim. By the time you hoist them back onto the deck everything else will have drifted away or gone do DJ's locker anyway so it really does not matter so much what you threw out there; it does matter that what you threw was enough in volume and distinguishable from the wind swept seas to find and keep in your eyesight. I don't care what exactly it is that you send overboard; just be sure that there is enough "stuff" out there that you can recognize a pattern of junk on some distant wave face with your binoculars should you need to widen your search.
What matters most is being able to locate the person who went in. If you are over 2 wave crests away from them I bet you would have an incredibly difficult time finding them; that's where a field of debris can help because if you sail below them and find a piece of debris (even lowly paper or styrofoam plates) you know you are somewhere within range of their position. The other issue is seeing something that is in the trough while you are on the wave crest. What you can see while on the crest is just the other wave crests and maybe a wave face or three on each side. That means that 90% of the sea surface in your visible range is not visible by your eyes.
You have to think in terms of search/rescue; not strictly "rescue because I'm in a position right now to pick them up". What if you miss them on your first attempt and then lose sight? What if the MOB pole loses it's pole end/flag? What if you can't find them before nightfall (and they went in at dusk)? What if your chartplotter shut off while starting the engine and you lost the MOB waypoint?
It's not about "what is the most environmentally friendly thing to do"; its about "what do I need to do to be able to find this postage stamp sized spot on the ocean again?" Think about it. Decide what the best and most visible things to throw in are before you leave the dock so that they are nearby if you need them. Forget about what is good for mother earth; you are trying to save a human life. Would an air horn tied to the horsecollar be a good idea (should the MOB be able to swim to it)? Would a small Radar reflector on the MOB pole be a good idea? You bet...