That's understandable, but perhaps they could just give him an isolated PC, and let him pick some github projects he wants to clone, download those (after approval of course) and put them on the PC, let him work away on it for a while, perhaps allow periodic pulls from those repos, and then when he has something to contribute, give those to someone to review for anything disallowed (like a manifesto in the comments or whatever), and once they confirm it's just working source code, allow it to be contributed somehow to the project so they can accept it or not, understanding where it's coming from.
It would be a good project but you need people to review all code. I think effort/money could be spent on more pressing matters in this environment; open-source contribution is rather niche.
I'm sure they can find outsiders happy to review code contributions. Any decently-run OSS project would already do this, after all, so it wouldn't be hard to just contact them and find a person already active in that organization willing to do the review and communicate back to the prison, assuming it's a project that really would like some extra full-time help for the next ~50-75 years (not all might, but some definitely would).
What other "more pressing matters" could there be anyway? Making license plates? This guy is smart and experienced with software development. It would be a waste for him to do something as mundane as making license plates or digging ditches (I don't even think they use prisoners for that these days).
