Whether or not Boo Radley's post trial monologue will work or fail depends on how well it is written and how much detail is included. I think it is a great premise, though, and I think there is plenty to work with from the novel's post guilty verdict.
The first thing that could be included in the monologue is Boo's thoughts on Tom Robinson, the trial, the verdict, and even Bob Ewell. That would allow you to flow into the next major event of Tom trying to escape prison and being killed. Shortly after that came the Halloween pageant. Guaranteed Boo saw Jem and Scout dressed up and walking to the school that night. Perhaps he could reflect on his "friendship" with them over the course of the year. From there it would be easy to include details about his rescuing them from Bob Ewell, meeting the Finch family, and having the sheriff turn a blind eye to Bob Ewell's cause of death. Finally, you might want to include his thoughts about Scout walking him home that night.
I agree with Sciftw that you have a great premise. It seems to me that your biggest challenge would be to show how Boo Radley, who is confined to his house all the time, could have known what was going on during and after the trial. I think your idea would "work or fail" depending on whether you persuade your reader that Boo Radley has access to substantial information about the outside world. The only explanation that comes to mind is that he overhears his father talking about the case with visitors. No doubt there would be plenty of gossip going around all over town--if Boo had opportunities to overhear it. He is an extremely isolated character. He is actually invisible up to the climax. I think many readers would like to know more about him. He is not very articulate, either. Your monologue would probably have to show that he has a unique way of thinking and expressing his thoughts and feelings to himself. You might refer to Benjy's stream-of-consciousness monologue in William Faulkner's novel The Sound and the Fury. If you are writing a monologue, then Boo has to be talking to somebody or else talking to himself. It occurs to me that he might be talking to Scout as she accompanies him back to his home. She would just be listening. This would be "in character" for her because she is a great observer and listener.