Asp.Net MVC ValidationSummary HTML error message - asp.net-mvc-5

Question

I've been asked to include a link in an error message when the Email address for a registration is already in use.
The validation for this property is done with a IValidatableObject.Validate function on the model. My validate function looks like so...
Public Overridable Function Validate(validationContext As ValidationContext) As IEnumerable(Of ValidationResult) Implements IValidatableObject.Validate
Dim results = New List(Of ValidationResult)()
....
If Not EmailAvailable(Email) Then
results.Add(New ValidationResult("The email address is not available. Forgot Password?", {"Email"}))
End If
Return results
End Function
In my views, I'm using a custom "MyValidationSummary" extension function to format the errors nicely.
The extension function does this...
....
Dim ul = New TagBuilder("ul")
For Each key In helper.ViewData.ModelState.Keys
For Each e In helper.ViewData.ModelState(key).Errors
Dim li = New TagBuilder("li") With {
.InnerHtml = helper.Encode(e.ErrorMessage)
}
ul.InnerHtml += li.ToString()
Next
Next
container.InnerHtml += ul.ToString()
Return New MvcHtmlString(container.ToString())
I know I could just remove helper.Encode, and just output the message as raw html, but this feels a bit hacky.
I'm trying to find a nice way to be able to selectively include html in the messages, while still retaining the default behaviour of encoding plain text messages.
What I thought of doing, is create a custom ValidationResult class, which optionally would include a HTMLString property, so that I can, if I choose, include HTML in the messages.
I can do this, but I don't know if there is any way to get at this custom ValidationResult from MyValidationSummary.
update:
For the time being, I've just added a placeholder tag into the error message, which I then substitute with the actual link in my MyValidationSummary extension method. It's very hacky, but it will work until I've found a better way to do it.