There is an obvious way to do this that doesn't impose high regulatory costs. A simple rule: The customer (and their independent mechanic) has access to anything the company has access to. Now you're not forcing them to write new service documentation, only to not restrict documentation they wrote anyway to their own dealers. You're not forcing them to support third party replacement parts, only preventing them from inhibiting it through software locks etc.

You don't have to force them to do anything, all you need is for them to not prevent others from doing certain things. Which is easy, because it's preventing documentation from being copied around or preventing independent third parties from making compatible replacement parts which requires active effort.