Interesting reading, especially this
article. Live or frozen is six one way, half dozen the other for the feeder. I don't think a feeder goldfish (or insert fish species here) cares if it's frozen before it gets eaten. If a fish even has the cognitive awareness of its existence (I don't think it does) to know that it's gonna get eaten, then it will probably have the awareness to know it's gonna freeze to death or suffocate. It will go through all the anxiety and fear and self loathing either way. To suggest that someone has a moral obligation to feed a captive piranha, a feeder you froze over a live fish is just plain silly. Either way the feeder dies and the piranha gets some calories.
I would think that a captive piranha would benefit from chasing and catching it's prey. They have the same sensory system that sharks have, enabling them to detect miniscule amounts of blood.
Owning a predator/scavenger fish means bringing a little piece of the wild and putting it in your house. If you're not prepared to see fish attacked and eaten by an animal designed for that purpose, don't buy this kind of fish, don't watch any nature documentaries on piranhas and don't eat a fish yourself.
If your argument is only about the way the feeder dies, then try and let it go. You're not a saint if you freeze a feeder first, or you feed a bite sized feeders instead of one larger one, or if you feed cod you bought at the grocery store. You're still putting a fish that was once alive into a tank with a predator, so that if it's not dead already, it soon will be.