I've just pushed the beginings of a Zero Hour reimplementation project to https://github.com/T...blyArmada/Thyme if anyone is interested in contributing to such a project.
The project is using a similar approach to the one recently used successfully by the OpenRCT2 project. That is, the reimplementation is initially being implemented as a dll that is injected into the game run time that replaces the games functions with new implementations as they are written, allowing it to be reimplemented a bit at a time.
I've already reimplemented the custom memory allocator the game uses as well as a couple of string classes.
Its very early days for the project, but if anyone has some C++ knowledge and is interested in getting involved, drop by the #thyme channel on freenode.net IRC. When some significant progress has been made it should become possible to fix engine bugs and add additional features to the game.