Just an update. I have conditional XML rendering working with XML::Generator like so:

augment 'as_xml' => sub {
    my ($self, $name, $opt)=@_;

    VendorAddRq(
        VendorAdd(
              Name($name),
              $self->maybeVendorTypeRef($name, $opt)
           ));
};

sub maybeVendorTypeRef {
  my($self,$name,$opt)=@_;

  return unless $opt;
  return unless my $v = $opt->{VendorTypeRef};

  VendorTypeRef(
        $self->hashrender(ListID   => $v) ,
        $self->hashrender(FullName => $v)
           );

}

sub hashrender {
  my($self, $key, $hash)=@_;
  return '' unless my $v = $hash->{$key} // undef;

  my $X = XML::Generator->new(pretty    => 2);
  $X->$key($v);
}
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