Not in the USA, anyway. Simply announcing that 'XYZ is no longer one of JWs' is not slander. Truth is an absolute defense to a defamation claim (libel or slander). Since XYZ was DF'd or DA'd, then XYZ is no longer a member of the religious organization known as JWs. Thus, the statement is true and there is no slander.
And to answer the question whether anyone has tried to sue the WTS for defamation--yes, they have. Olin Moyle won a libel lawsuit against the WTS for publishing defamatory things about him in the WT magazine.
In the 1980s, a DA'd witness sued the WTS for defamation. The 9th circuit court of appeals shot her down. The case is Paul v. Watchtower. A copy of the opinion is available on Barbara Anderson's excellent Watchtowerdocuments.com website: http://www.watchtowerdocuments.com/downloads/1987%20Paul%20Decision%209th%20Circuit%20Court%20Appeals.pdf
This case dealt with a DA'd person who was shunned by her former fellow Witnesses, so it's not exactly the same as the issue raised in this thread. But cases like this show that U.S. courts are unlikely to allow a plaintiff to succeed against a religious organization when the suit is based on religious practices--such as shunning and disfellowshiping.