Arxiv for sure. That was the first place I went, however no ability to download the entire database and scraping it would be a project on its own. It begs the question how can we as a civilization "own" our own data and not "rent" it through a website or paywall?
Which brings me to LexisNexus. That is a knowledge store we should all be able to access, crucial information about how the world works! It's only available for Governments and Law Firms...
In my hypothetical the Stackoverflow suggestion is on-point and would also be invaluable right in line with your AI strategem. We'd need some way to categorize, and order all that information to make it applicable would be the challenge I would think. Further, using your game theory of AI, it would need to also be in defense computer system ingestible maybe in real-time, so it would not be available/discoverable/decipherable/hackable up until the moment the data needed to be used. Some kind of OCR oracle for paper to digital "process" for AI defense.
I'm still following your breadcrumbs, I could say more!