...because @specialized is a public class and those are the types you have to give it. It's an annotation so your hands are pretty well tied.

You should be able to do something like

object Foo { final val MySpecializeTypes = Seq(Byte, Short, Long) }

But you cannot, because the inferred type of MySpecializeTypes is not List[SpecializableCompanion], and you can't even cast it because you can't reference the type.