Looking at the source, it doesn't look like you can configure it at all. It recreates the XML::LibXML object in various places, without any way of supplying your own parameters:

sub _parse_bytestream {
    my ( $self, $fh ) = @_;
 
    $self->{ParserOptions}{LibParser}      = XML::LibXML->new;
    $self->{ParserOptions}{ParseFunc}      = \&XML::LibXML::parse_fh;
    $self->{ParserOptions}{ParseFuncParam} = $fh;
    $self->_parse;
    return $self->end_document({});
}
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You'll have to patch the module and find out how to make XML::SAX::Base pass parameters down to the implementation, and then pass these parameters on to XML::LibXML.