With SSLv2 you will never be able to add a sig_len.

def addfield(self, pkt, s, val):
        """With SSLv2 you will never be able to add a sig_len."""
        v = pkt.tls_session.tls_version
        if v and v < 0x0300:
            return s
        return super(SigLenField, self).addfield(pkt, s, val)