You can - the data gets POSTed to a URL of your choosing, signed with your secret key. But I don't like the interruption in flow that I have to force on the us

You can - the data gets POSTed to a URL of your choosing, signed with your secret key. But I don't like the interruption in flow that I have to force on the user in order to present a second (nearly identical) registration form just because the validation on the first is rubbish - especially for a field as important as email address.