Just for my interest: How can a rebuild of a copyrighted ROM be without copyright?
The new ROM should do exact the same as the original one (if 100% compatibilty is reached). So it is still under the original copyright.
It is not the original but it does exactly the same things. This could be achieved by clean room reverse engineering with a test suite (test what a command does in the original environment and try to obtain a similar result with your own code) . The code itself would be written from scratch and could be published under a free license.
That's the same way that e.g. Wine uses to develop a free winapi.