Well, it could be done on both 32 and 64bit as long as we can provide some fallback for the old BeOS apps. We did the same for get_system_info, for example, when we removed the limit of 8 CPUs. BeOS apps will only see the first 8 CPUs, newly built apps will see all of them. We could do the same for dead keys.
However, I don't see if this is an improvement over the input method based solution. Not being a greek typer myself, I can easily miss somethng here, but I would expect the input method to give at least as good, if not better, results.
Then there is the question of integration with the OS. If the input method works well, then we should probably make it available in the default image, and enable it automatically if you select a greek keyboard?