First - its not a Mass. It’s the only day of the year when Mass is not offered. Second, although I don’t think its part of the service anymore, in the 1970s the liturgy still included prayer for the unjustly imprisoned. In the early 70s I went to Good Friday Mass with a man who had escaped from prison and had been imprisoned for giving flowers to Nancy Fonda in Canada when he was in uniform. He was on one side and a police officer with a gun was on the other side. I’ll never forget it. So these people were in a sense unlawfully imprisoned. If I had been there I would hope that I would have stayed.
You are correct that they don’t include a specific intercession prayer for those unjustly imprisoned.
However, it is mentioned in the 10th intercession for those in Tribulation that they be “unlocked from prison,” and mentions those enslaved, suffering hunger and disease, ect.
For reference, the Roman Missal third typical edition.