Found a way by implementing a custom IMetaDataProvider
Seems to work fine.

 public class CustomEpiserverMetadataProvider : IMetadataProvider
 {
     private IMetadataProvider _defaultMetadataProvider;

     public CustomEpiserverMetadataProvider (IMetadataProvider metadataProvider)
     {
         _defaultMetadataProvider = metadataProvider;
     }

     public ExtendedMetadata CreateMetadata(IEnumerable attributes, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName)
     {
         return _defaultMetadataProvider.CreateMetadata(attributes, containerType, modelAccessor, modelType, propertyName);
     }

     public IEnumerable GetMetadataForProperties(object container, Type containerType)
     {
         var metadataForProperties = _defaultMetadataProvider.GetMetadataForProperties(container, containerType).ToList();

         if (container is PromotionData)
         {
             foreach (var propertyMetadata in metadataForProperties)
             {
                 if (propertyMetadata.PropertyName == nameof(PromotionData.Description))
                 {
                     propertyMetadata.UIHint = "my-custom-ui-hint"; // Replace the default UIHint "textarea" with our custom editor
                 }
             }
         }

         return metadataForProperties;
     }
 }

Registering it using a InitializationModule

  [InitializableModule]
  [ModuleDependency(typeof(InitializableModule))]
  public class CustomEpiserverMetadataProviderRegistration : IInitializableModule
  {
      public void Initialize(InitializationEngine context)
      {
          var registry = context.Locate.Advanced.GetInstance();
          var metadataHandlers = registry.GetMetadataHandlers(typeof(ContentData));
          IMetadataProvider defaultMetadataProvider = (IMetadataProvider)metadataHandlers.FirstOrDefault(i => i is IMetadataProvider);

          registry.RegisterMetadataHandler(typeof(ContentData),new CustomEpiserverMetadataProvider(defaultMetadataProvider));
      }

      public void Preload(string[] parameters) { }

      public void Uninitialize(InitializationEngine context) { }

  }