Looking further into it, I managed to get the Trigger category to show up by doing this:
UCLASS( ClassGroup=(Custom), meta=(BlueprintSpawnableComponent), Blueprintable, ShowCategories = (Trigger))
and this:
/**If True, only specifc actors can trigger. */
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Trigger)
bool bTriggerWithSpecificActors;