Hi Nathan,

Its possible yes. Actually very easy.

-Create a helper class like this. You can pretty much copy this and rename where appropriate.

 using Umbraco.Core;
 using Umbraco.Core.Composing;

 namespace Website.Core.Helpers
{
    public class MyTestHelperComposer : IUserComposer
    {
        public void Compose(Composition composition)
        {
            composition.Register<IMyTestHelper, MyTestHelper>(Lifetime.Singleton);
        }
    }

    public interface IMyTestHelper
    {
        string MyTestMethod();
    }


    public class MyTestHelper : IMyTestHelper
    {

        public MyTestHelper()
        {

        }

        public string MyTestMethod()
        {
            return "some value to be returned";
        }
    }
}

-Then you can inject your custom helper in the exact same way you do the Umbraco helpers and services.

Eg.

public class BlogDetailsPageController : RenderMvcController
    {
        private readonly IMyTestHelper _myTestHelper;
        public BlogDetailsPageController(IMyTestHelper myTestHelper)
        {
            _myTestHelper = myTestHelper;
        }

        [HttpGet]
        public override ActionResult Index(ContentModel model)
        {
            var vm = new BlogDetailsPageViewModel(_siteHelper, _membershipSubscriberHelper, model.Content) { };

            string test = _myTestHelper.MyTestMethod();

            return View("/Views/blogDetailsPage.cshtml", vm);
        }
    }

Hope this helps.

Regards

David