If the topic of the lawsuit is going to be discussed, then the invited TSA agent's testimony at the hearing could be relevant to the lawsuit regardless of whether Schneier is there. If the topic of the lawsuit is not going to be discussed, then everything is OK. Remember congress people will do the questioning, not Schneier or Schneier's council. Perhaps Schneier's mere presence could inject some lawsuit-related questions (using a zero-day legal flaw the congress-testimony process, perhaps?) but this possibility seems remote.