I’m not sure I follow the suggestion.

• NCMEC, the US-based organization tasked with fighting CSAM, has already partnered with a list of groups to develop CSAM detection tools. I’ve already linked to an overview of the resulting toolsets in my original comment.
• The datasets used to develop these tools are private, but that’s not an oversight. The datasets are… well… full of CSAM. Distributing them openly and without restriction would be contrary to NCMEC’s mission and to US law, so they limit the downside by partnering only with serious/capable partners who are able to commit to investing significant resources to developing and long-term maintaining detection tools, and who can sign onerous legal paperwork promising to handle appropriately the access they must be given to otherwise illegal material to do so.
• CSAM detection tools are necessarily a cat and mouse game of CSAM producers attempting to evade detection vs detection experts trying to improve detection. In such a race, secrecy is a useful… if costly… tool. But as a result, NCMEC requires a certain amount of secrecy from their partners about how the detection tools work and who can run them in what circumstances. The goal of this secrecy is to prevent CSAM producers from developing test suites that allow them to repeatedly test image manipulation strategies that retain visual fidelity but thwart detection techniques.

All of which is to say…

… seems like law enforcement would have such a data set and seems they should of course allow tools to be trained on it. seems but who knows? might be worth finding out.)

Law enforcement DOES have datasets, and DO allow tools to be trained on them… I’ve linked the resulting tools. They do NOT allow randos direct access to the data or tools, which is a necessary precaution to prevent attackers from winning the circumvention race. A Red Hat or Mozilla scale organization might be able to partner with NCMEC or another organization to become a detection tooling partner, but db0, sunaurus, or the Lemmy devs likely cannot without the support of a large technology org with a proven track record or delivering and maintaining successful/impactful technology products. This has the big downside of making a true open-source detection tool more or less impossible… but that’s a well-understood tradeoff that CSAM-fighting orgs are not likely to change as the same access that would empower OSS devs would empower CSAM producers. I’m not sure there’s anything more to find out in this regard.