According to a recent decision issued in United States v. Jesus-Casteneda, it is now constitutional for witnesses to testify in disguise. 
Apparently, a “confidential informant” was allowed by the trial court to testify in a wig and mustache. The prosecutor trying the case asked the trial judge for permission to allow him to disguise himself out of fear of retaliation. The trial judge allowed the testimony to be given in the disguise.
The defendant argued on appeal that the testimony violated his constitutional right to confront the witness. He argued that to allow a witness to testify in a disguise would suggest that the witness has something to fear by participating in the trial. The defendant, an accused member of a drug cartel, was apparently concerned that the disguise would cause the jury to draw a negative inference regarding him.
The Ninth Circuit Court of Appeals held that the disguise was reasonable under the circumstances and furthered an important governmental interest, namely the safety of a witness.