I don't think that Michael would have been entitled to a disclosure of subsequent witnesses. He wouldn't technically be a target of the investigation. The committee would have to target "Organized Crime Penetration of Interstate Commerce" or some such. It would have to be something plausibly related to defined Congressional powers.
It almost would have to be this way: most Congressional hearings are essentially fact finding missions, they can continue for long periods of time, and a committee often won't know who subsequent witnesses will be until they hear what this witness has to say.
Since we don't know the content of the Senate resolution that established the committee, we can't say for sure what its jurisdiction was. But it is obvious that the hearing was an adversary one. Given that Pentangeli's verbal testimony contradicted his written deposition (as the Committee chair pointed out), we know for sure that he was deposed previous to the hearing, that therein he accused Michael of crimes, and therefore, Michael was a target of the committee's investigation. I think disclosure would have applied.
