The OP's ethical dilemma is how to achieve the end goal of feeding his fish in a manner which is ethical. The fact that he seeks a way to make live feeding "more ethical" indicates that his personal ethics are offended by it.
The suggested solution, i.e. sedated feeders (!), is so convoluted and so rife with concerns, both ethical and practical, that it is difficult to believe that this entire thread is not some sort of elaborate joke.
The way to make live feeding completely ethical, by virtually anyone's standards, is quite simple: don't do it. It's that simple. Don't do it for the entertainment that it seems to offer to some people...don't do it because of the health concerns of disease and parasite infection...don't do it because it is an inconvenient PITA...and most importantly, don't do it because it is virtually never required!
I use the term "virtually" only because there are exceptions to every rule...never say never. But in almost 6 decades of fish keeping, much of it heavily invested in big predatory fish and big tanks...I have never encountered a fish species or individual that could not be weaned onto frozen food or even dry pellets with little effort, certainly none that required more than a few weeks to do so and usually far less time. In fact, the most challenging "live food or nothing!" species are tiny micropredators like Seahorses and the like. Big piscivores and other large predators are never a problem.
No, you can't necessarily just starve 'em for a few days and then drop in a handful of Massivores; that sometimes works, but it's often required to actually do a little bit of creative thinking to fool them, or to manipulate their behaviour in the direction in which you want it to go. That's part of the joy of fishkeeping.
It certainly beats keeping a bucket of goldfish and trying to drug them or hypnotize them or brainwash them or some other such nonsense.