Personally I think that saying that the DCI4 spec is "confidential due to security concerns" is just a load of nonsense. There's nothing very special in it. I suspect it was more that Acorn never felt it worth the effort involved in tidying it up into a publically releasable form; anyone who needed it could always get it by asking (and still can I'm sure).
Still, now that ownership of RISC OS has changed, it might be worth asking Castle to finally make it generally available.
I would also advise looking at FreeBSD as a first port of call for adapting device drivers. As the rest of the Internet stack is BSD-derived there are some advantages, though the DCI4 interface to the OS is rather different compared to both BSD and Linux.