@Francesco Sechi: Would it not just require a minimal change to the
_write_data() method? Something along the lines of (sorry, no Python
expert, maybe I am way off):

def _write_data(writer, data, is_attrib=False):
    "Writes datachars to writer."
    if is_attrib: 
        data = data.replace("\r", "
").replace("\n", "
")
    data = data.replace("&", "&amp;").replace("<", "&lt;")
    data = data.replace("\"", "&quot;").replace(">", "&gt;")
    writer.write(data)

and in Element.writexml():

    #[...]
    for a_name in a_names:
        writer.write(" %s=\"" % a_name)
        _write_data(writer, attrs[a_name].value, True)
    #[...]