A year ago, I had this fun comment about if necromancy were possible in a courtroom setting. And I think the follow-up is relevant here:

TL;DR: rules of evidence would still apply to the undead, and judges must take care to balance the probative value of evidence with any prejudicial quality it may carry. (to be abundantly clear, this was a schittpost lol)

even when you have a live body on the stand about to give testimony, it is essential to lay the foundation as to who they are and their legitimacy.

If a masked vigilante’s legitimacy as a vigilante cannot be proven independently – a tough act since they would want to maintain their secret identity, coupled with the possibility of copycats or false flag operatives – then a jury or the bench would be reluctant to give their testimony much value. It’d be no different if you or I waltzed into court and proclaimed to be the world’s foremost expert on bitemark analysis and underwater basket weaving, but that you just have to “trust me, bro” on that claim. No reasonable person would believe that claim.

Now, your question focuses on whether a masked vigilante can testify but with a proviso to protect their interests. To that question, the answer is: maybe. Sometimes a witnesss (eg FBI Special Agent) can have their identity protected being exposed in open court, but still allow all parties (clients and their attorneys) to know the witness’s identity, for the purpose of fact checking. While there’s a fairly strong interest that trials be mostly open, a Special Agent’s identity should generally be protected, since that could be the very essence of their occupation.

And you could maybe say the same thing for a masked vigilante. But in the opposite, one could argue that the general public has an overwhelmingly strong interest in learning the identity of a masked vigilante, which might suggest that the court deny the vigilant the benefit of a private testimony.

Still, the vigilante could proceed to give testimony, unmasked if so needed. However, if the question was tweaked to be “can a court force a masked vigilante to testify, and thus reveal their identity?”, the answer in USA law is no.

Specifically, the Fifth Amendment guarantees that no one can be forced to testify if the testimony can be used against them later. So either the testimony isn’t taken at all, or that they are immunized for anything that they testify about. Those are the only two remaining options that satisfy the 5th Amendment. And so, since a masked vigilante would have potential criminal liability for past crimes, the government (federal or state) cannot force them to answer questions on the stand, without first granting immunity. If they offer no immunity, the vigilante can simply reply that they’re invoking their right from the 5th Amendment, and no harm can come to them for doing so; being harmed for invoking a right would make said right into a meaningless thing.

But if the prosecutor decides that it’s super important to hear what the vigilante has to say, then they can grant immunity. Unlike a pardon which is a gift that a person can choose to decline, immunity is unilaterally granted, and the recipient cannot reject it. Once that’s done, someone can be forced to testify (eg locked in jail until they will talk), since there is no longer a risk of the testimony coming back to harm themselves later.

Note: those words could be entirely damning to somebody else, and there’s no such thing as “third party Fifth Amendment” rights. This is almost always the reason why the prosecutor would grant immunity, to get the compelled testimony from underlings or witnesses necessary to convict a bigger target. If a masked vigilante was the only witness to documents about accounting fraud, which were later burned, that could be really useful when trying to pursue white collar crime, even if the cost is to give up any possibility of prosecuting the property damage crimes that the masked vigilante may have committed.