What kind of sorting do you mean, sorting the powerball from the other balls with sensors, or just managing multiple balls within the machine? STTNG is a champ at diverting balls from various holes to VUKs under the playfield. You have one so I assume that's the first thing you imagined

Avatar LE and Rolling Stones LE are supposed to do this as well to detect the non-metallic ball but I have not seen them in action.

Zitt,

TZ doesn't really "sort" the balls.
It attempts to keep track of where the Powerball is using eddy sensors, and tracking logic.
Goldball somehow identifies the "gold" ball, but I never looked at the mech that does that.
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Avatar has a white ceramic ball. It doesn't sort it. It just knows when its being used.

Right. There are two strips of metal on the shooter lane. A metal ball completes a circuit when sitting on them. Ceramic doesn't. So with the shooter lane switch activated the machine can tell what kind of ball it's launching.

Having never played an Avatar (crap.. I haven't even seen the movie..) this is a neat idea. Assuming the metal doesn't corrode or wear or gunk up, probably an easier detection method than TZ..

Still need to do an experiment with a reed-switch nearby the balltrough of the Twilight Zone.

AFAIK No games actually "sort" balls, Pretty sure Gold Ball has two separate ball feed positions and the gold finish on the ball is just for show. The rest of them just detect when the special ceramic ball comes into play.

For materials sensing your options are a reed switch or some sort of hall effect or eddy sensor, or a switch closure like it sounds like Avatar LE might use, not familiar with it. If the ball is stationary than an albedo sort or color detection would also probably be possible with an RGB led and one or more cheap phototransistors.

Another option: If you're already going to be getting custom ceramic balls or something in any sort of quantity, it's possible that you could get them with a passive RFID embedded in them. No idea what that would cost you. With a low power antenna you could limit the effective range to a couple inches. That would be sweet.