In theory (not tried) you should be able to do it just using the hook: integrate_register_check
That hook passes two arrays: $regOptions and $reg_errors
Then you can do something like:
function only_valid_emails($regOptions, $reg_errors)
{
// if $regOptions['email'] is NOT valid
// $reg_errors->addError(array('email_notallowed', array(htmlspecialchars($regOptions['email'], ENT_COMPAT, 'UTF-8'))), 1);
}
And then you define "somwhere" a $txt['email_notallowed'].
ETA: and if it doesn't work...it's probably a bug!