The Judge might throw you out of the court for attempting to unduly influence the jury. Besides, it would be highly unrecommended for witnesses to wear such shirts anyway because it would make them appear biased and ruin their credibility with the jury. I don't see how this could actually benefit the defendant, but you could cause him harm. Your presence alone is sufficient evidence of your support. You should run this idea by his defense attorney.
DISCLAIMER: This answer is provided as general information, which may not be appropriate for the specific facts of your particular situation. No attorney-client relationship has been established based on this limited communication. You are advised to consult with an attorney in your jurisdiction before taking any action or inaction that may affect your legal rights. www.hecklerlawoffice.com
There have been a lot of cases that question the line between freedom of speech and the court's authority to maintain decorum in the courtroom. The fact is you will not have an attorney to represent your interests, and the defendant doesn't really have standing to fight for your right to wear such a shirt through his attorney. It is likely that, right or wrong, the judge will kick you out of the courtroom. Further, as a criminal defense attorney myself, if one of my client's friends asked me if they could wear such a a shirt I would ask them not to as I think it looks like a cheap tactic that a jury would probably not appreciate.
Notice: The information contained herein is intended as general legal information and does not create an attorney-client relationship and is not governed by confidentiality rules. This general legal information is not a substitute for seeking the direct advice of legal counsel.
I don't see why you wouldn't be able to wear such a t-shirt. Of course, some judges have certain rules when it comes to decorum in the court room. You should check with the court house to be sure. Your best course of action would probably be just to dress respectfully for the hearing.
I never understood why some defendants show up to court wearing t-shirts depicting marijuana or similar images. If you are arrested for a gun related crime, you should probably reconsider the Scarface t-shirt. Judges and prosecutors are human. You have one chance to make a first impression.