Not sure what "once in a while" is - it would probably be more zoas than you can grow, and an expensive diet : ) You can train most fish to take a prepared diet, if you are patient and take the effort to do so. But, if a fish is a corallivore, it will likely eat corals every once in a while even if you actually have success training them on frozen, pellet, etc. (which is harder likely for the butterflies you mention)The issue with butterflies often is keeping them healthy long term, and getting them to eat prepared foods early enough so they survive through the first month or two. Here's a good video of a Bob Fenner lecture about butterflyfish:
Typically the butterfly fish you mention are not kept in reef tanks, but in fish-only systems with no corals. There are always exceptions, but most people don't want fish eating their precious corals : ) And this begs the question as to how and if an obligate corallivore should be kept in a system without their typical foods. Hence the caution and stereotypical bad survivability with a lot of butterflies.
