>Hadn't come across this but it appears you can override the default validation by specifying your own class derived from RequestValidator:
>http://msdn.microsoft.com/en-us/library/system.web.configuration.httpruntimesection.requestvalidationtype.aspx
>Just a question of overriding IsValidRequestString (and maybe supplying validationFailureIndex).....

I think you wanted to reference to this page instead:

http://msdn.microsoft.com/en-us/library/system.web.configuration.httpruntimesection.requestpathinvalidcharacters.aspx

Is that correct?