Apples old hardware isn't all that closed - most models have enough exploits it's very possible to get your own low level code running.

The key issue is that hardware drivers aren't there, and that requires a massive amount of reverse engineering work.

Apple could release documentation for the hardware, making things easier, but from apples point of view that opens them up to patent litigation (you can be sure trolls will scour the documentation for evidence of violated patents), and most internal documentation isn't of sufficient quality to release without at least someone reading it all over and removing swearwords...

Collecting the documentation together will be hard - it is probably scattered amongst hundreds of git repos, wikis, documents, spreadsheets, photos of whiteboards, on some engineers laptop, and mixed in with code and secrets.

Even then, without a lot of people putting a lot of effort in, you wouldn't see desktop linux running...

Overall, I can see why Apple doesn't help these guys - the disadvantages outweigh the advantages.