I've heard that if something is not prohibited, then it is allowed...
As a general principle of law, that's correct.
...which would seem to imply that I do have a right to let's say videorecord the deposition.
You are assuming that no law prohibits the recording. That's not correct. Maryland's Courts and Judicial Proceedings Code section 10-402(a)(1) states:
"(a) Unlawful acts. -- Except as otherwise specifically provided in this subtitle it is unlawful for any person to:
(1) Willfully intercept, endeavor to intercept, or procure any other person to intercept or endeavor to intercept, any wire, oral, or electronic communication."
This is a criminal offense. There are several exceptions in the statute, most of which apply to law enforcement. But there is one exception that applies to everyone in paragraph (c)(3):
"(3) It is lawful under this subtitle for a person to intercept a wire, oral, or electronic communication where the person is a party to the communication and where all of the parties to the communication have given prior consent to the interception unless the communication is intercepted for the purpose of committing any criminal or tortious act in violation of the Constitution or laws of the United States or of this State."
Thus, as I said before, if you have the consent of ALL parties, the taping would be permitted. If they don't agree to that, you can't.
In addition, as gemini47 noted, the deposition is done under the authority of the court. You've been issued a subpoena to attend and give testimony and you do not get to show up and then condition your participation on being able to record the deposition yourself. If they don't agree to let you record and you refuse to participate, you may be facing a contempt problem with the the court.