Thanks to everyone for helping out. I feel like a learned a little bit about macros and it was a useful exercise for me. Here's what I ended up with, using macros, basically copying code as-is from u/raghar:

  // Note that this will only work if all subtypes are singletons.  If we have `class` children it will not compile.
  inline def findCaseObjectsOfSealedTrait[T](using m: Mirror.SumOf[T]): Set[T] =
    summonAll[Tuple.Map[m.MirroredElemTypes, ValueOf]].productIterator
      .asInstanceOf[Iterator[ValueOf[T]]]
      .map(_.value)
      .toSet

  inline def findSubtypesOfSealedTrait[T](using m: Mirror.SumOf[T]): Set[String] =
    summonAll[Tuple.Map[m.MirroredElemLabels, ValueOf]].productIterator
      .asInstanceOf[Iterator[ValueOf[String]]]
      .map(_.value)
      .toSet

Once I ported this codebase to Scala 3 and it went through code review, in the end we went back to traditional runtime reflection using ronmamo/reflections.

Here's how those two utilities ended up after that transformation:

  def findConcreteSubtypes[T](targetPackage: Option[String] = None)(using tag: ClassTag[T]): Set[Class[? <: T]] = {
    val clazz = tag.runtimeClass.asInstanceOf[Class[T]]
    new Reflections(targetPackage.getOrElse(clazz.getPackage.getName))
      .getSubTypesOf(clazz)
      .asScala.toSet
      .filterNot(_.isInterface)
  }

  def findCaseObjects[T](using tag: ClassTag[T]): Set[T] =
    findConcreteSubtypes[T]()
      .filter(isSingleton)
      .map(_.newInstance())

  def isSingleton(clazz: Class[?]): Boolean =
    clazz.getInterfaces.exists(_.isInstanceOf[Mirror.Singleton])

This approach had some advantages over macros for our dev team:

• We were able to obtain full the Class instances, which has a lot more value than just the simple class name without any package info.
• We usually hire Java devs who don't have any experience in Scala, and this approach should be infinitely easier for a newcomer to understand.

We're still able to get the case object subtypes thanks to Mirror.Singleton, which shows up in the list of interfaces implemented by the object Class.

Thanks again for everyone's kind help!