And mostly pointless The spec would contain nothing that CryEngine's marketing page doesn't contain. Keep in mind, you can't write anything specific with regards to the actual implementation. You can just provide a description of the interface, you can get that one for much cheaper already.

A clean room approach is great for when the interface isn't known and you want to create something that is compatible, you can have someone reverse engineer the interface and write neatly down how the things fit together and what should happen if you provide input X. But you still need to figure out how to get from input X to the desired result yourself, and that most likely takes a lot of actual engineering effort.