@Henry: "Confesar a alguien" is used only in the sense that a Catholic priest will hear someone's sins (there is also a figurative use of this formula, but here, it's not the case).
When the police hears a confession, or when someone avows something to someone else, then the subject confessing is clear.
- María me confesó que está enamorada de Pedro.
- Te confieso que me gustan mucho los chocolates.
- El ladrón no quiere confesar que mató al dueño de la tienda.
- Police detective talking: "¿Vas a confesar de una vez?"
The whole stanza of this song goes like this:
Al preso número nueve, ya lo van a confesar
Está encerrado en su celda con el cura del penal.
Y antes del amanecer la vida le han de quitar
Porque mató a su mujer y a un amigo desleal.
The "they" from "ya lo van a confesar" is, more than a subject, an impersonal form, a generalization, that somehow conveys that when anyone is in jail, and will confess to a priest, it's because they will be executed next.
As far as I know, the verb works the same way in English; for example in "the priest confessed the criminal", so your first proposal would work alright if the next line wouldn't say who will hear his sins. Probably a better translation would be something similar to: "the prisoner is going to be confessed". The passive voice in Spanish is not heard as often as in English, but we do use plenty of impersonal forms, whose equivalent in English is commonly the passive voice.