Oh the adventures of JavaScript and ASP.NET.
Can you turn on validators [RequiredFieldValidators or otherwise] from JavaScript. Yes, and actually it is easier than I thought.
ValidtorEnable(ControlId,True/False)
Ex:
//You could easily make this prettier by not hardcoding the ClientID here
ValidatorEnable(document.getElementById('ctl00_ContentPlaceHolder1_Login1_UserName'), true);
Keep in mind, you need to pass the
object not the clientID of the object.
Source:
http://msdn.microsoft.com/en-us/library/aa479045.aspx