>>You can specify the replacement validation in web.config:

<configuration>
>>  <system.web>
>>    <httpRuntime requestValidationType = "{namespace}.CustomRequestValidation,{assemblyname}"/>
>>  </system.web>
>></configuration>

>
>Thanks, but if I would like to keep control at the framework level, this seems like the place to put it. Here is the top of the GlobalBase class which inherits from System.Web.HttpApplication.
>
>

>Imports System.Reflection
>Imports System.Web
>Imports System.Web.Util
>
>Namespace Framework
>
>    Public Class GlobalBase
>        Inherits System.Web.HttpApplication
>
>        Private oApp As Framework.App
>
><ConfigurationPropertyAttribute("requestValidationType", DefaultValue := "System.Web.Util.RequestValidator")> _
><StringValidatorAttribute(MinLength := )> _
>Public Property RequestValidationType As String
>            Get
>                Return "CustomRequestValidation"
>            End Get
>        End Property
>

>
>However, when doing so, the ) in the MinLenght definition is highlighted and the designer gives:
>
>"Expression selected"
>
>I do not know why it is complaining with the default.

Sorry, I should have looked more closely at your code.
You can't use it in that way. As per the link RequestValidationType Property is simply defined as an attribute of HttpRuntimeSection:
http://msdn.microsoft.com/en-us/library/system.web.configuration.httpruntimesection.requestvalidationtype.aspx
You need a class such as this:

Public Class MyRequestValidator
	Inherits System.Web.Util.RequestValidator
	Protected Overrides Function IsValidRequestString(context As HttpContext, value As String, requestValidationSource__1 As System.Web.Util.RequestValidationSource, collectionKey As String, ByRef validationFailureIndex As Integer) As Boolean
		validationFailureIndex = 0

		'Spurious rejection test
		If requestValidationSource__1 = RequestValidationSource.RawUrl AndAlso value.Contains("Default") Then
			Return False
		End If


		Return MyBase.IsValidRequestString(context, value, requestValidationSource__1, collectionKey, validationFailureIndex)
	End Function
End Class

and then reference that in web.config.....