I got an email from Steve Maine who used JavaScript's support for function pointers to extend the Page_ClientValidate function. Steve's version calls the built-in version, but then sets the cursor focus to the first invalid control (assuming there are any). Pretty cool.
However, Steve goes on to talk about how quickly function interception can become unmaintainable. Dealing with the ability to override expected functionality is hard enough in object oriented languages. They call it the "fragile base class problem" for a reason. In JavaScript, you have the potential for the "fragile intercepted method problem". Granted, my solution is also "intercepting" a method, albeit in a lo-tech, code injection, search-and-replace kind of way. But maintainability needs to be of paramount importance in system design. I don't want to have to go mucking about in the plumbing primarily because plumbing skills are at a premium (I hear Don is trying to hire them all for his team).