I medieval law there was no protection against bearing witness against yourself- even under torture if necessary. Tribunals make limited exceptions to the limitations of using magic to observe magi's activities for investigators under limited circumstances- they are far more liberal than secular courts in part because they are effectively all part of the "noble" class of their society (grogs just get punished as the mage or covenant sees fit), so it would have to be fairly extraordinary circumstances to completely remove their legal protections against magical investigation, but it could happen. I suspect torture would even be more rare despite not being forbidden by the code.
Seeing as Hermetic trials aren't really like a modern trial, but are more like a debate carried out in front of the voters at Tribunal with the prosecution presenting their case and the defence having to publicly justify themselves, then anything goes. Both sides are trying to convince the voters of the strength of their argument.
You can remain silent if you want, but it's entirely up to the prejudices of the other magi at Tribunal whether this counts against you or not. The accuser and accused can employ every rhetorical trick they like to trip the other side up. With enough friends or sufficient bribes, it's possible for the innocent to be railroaded and the guilty to walk free.
I've always imagined hermetic trials to be similar to the ancient Greek trials, where the defence and prosecution had a set time to engage in oratorical debate trying to make their point and convince the jury (or the tribunal)
My idea of a hermetic defence would be something along the lines of the piece by Antiphon called "On the Murder of Herodes"
In "antagonists" they talk about medieval courts. Baron le cornu shows a civil court; the quaesitor shows a hermetic court.
In a civil court, there is no presumption of innocence and no protection against self incrimination. There also isn't worry about conflict of interest. The judge commonly stood to benefit personally from a guilty verdict.
In a hermetic court, you can refuse to answer a question on the grounds of irrelevance but not self incrimination.
To be fair in Hermetic court you can refuse to answer a question on teh grounds that you are having a snit and don't like the tone of the prosecutor's voice. Might not work to your advantage, but you can do it.
All of the "rights" written in modern constitutions and laws are written for a reason. Most of them were written to repudiate customs or laws that went before. Prior to modern and constitutional law, you were required to testify against yourself, ex post facto laws not only existed, they were the way things were done, and you had no freedom of speech, religion, arms, etc. You get the picture. If it is a written right or freedom now, assume it didn't exist before it was written down.
Late, but I would say that a defendant cannot be compelled to dispute the facts that the prosecuting principle sets forth. If a defendant doesn't testify, then the prosecutor's word on the matter is taken as a given.