My opinion has always been that if a brother has a concern or information about a potential candidate then he should approach a member of the investigation committee about it, who can then determine it's validity. I don't know if this is legal either, but it seems better to me than open discussion.
And word could still get back to the potential candidate. Probably less likely, but easily possible.
There is a provision where, before the initiation itself three brothers can go to the WM and protest the ballot. From the law book:
Article 423 When and By Whom
Any member of a Lodge in this jurisdiction may protest, either orally or in writing, a candidate for any degree, either before or after his election. In each case, the protester must give clearly his name and the name, number, and location of his Lodge.
More guidelines that you can look up yourself.
To be effective as to any petitioner for a degree, protests must be made after the petition is received by the Lodge and before conferring the degree has begun.
Article 425 Effect of Protests
A protest shall have the same effect and value and be counted as a blackball under Art 418. Three protests, or a combination of protests and blackballs with a combined total of three shall reject for one year. Four protests, or a combination of protests and blackballs totaling four shall reject the petitioner for two years;
More rules you can look up yourself.
So there is a protest. And once again, one that could easily get back to the candidate if a candidate's buddy is supporting the candidate, and the protesters are staunchly against the candidate.
The censor on discussion also prevents positive talk regarding a candidate. So if the general vibe is that a petitioner is going to get blackballed for an unjust reason, but you have some important, positive things to say about a candidate, you can't do anything about it.
I know I'm speaking in hypotheticals. But I think it's worth discussing.