DD has/had a sunburst recently that was eating. I ordered a dusky and a rainfordi from DD and LA respectively just before christmas. The Dusky came in eating, just as advertised from DD. The rainfordi took some work, but he eats most of the foods I offer readily, more so than the dusky does now.
My advice is to have a lot of clam on the half shell available, that is both of these fishes preferred food. They also eat the LRS foods pretty well. Both of mine went through a period where they weren't eating much of anything that was free floating and someone on RC recommended finely chopped uncooked table shrimp - this is also a good thing to have on hand. I like the wild caught ones if you can find them.
I've not kept a Sunburst, but it's one I would definitely try. I'm not convinced that the corallivores necessarily need the protein provided by the corals they eat in the wild, so much as they just need a ton of food through out the day.
Quarantine would be a definite must, and I might even try adding another fish that you know will eat a clam, so it kind of teaches the sunburst what to do. Conversely, sometimes a fish like this is better without any feeding pressure too, so you'll just have to feel that strategy out. Maybe start him of in quarantine by himself and if he's not eating anything after a couple days, add one of your current fish that would be compatible and that you know would go after the clams.
Let us know how it goes, good luck!