We face problems like this all the time in VFX, where the model is simply too big to sculpt in one go. So.....the simple solution is to basically create a duplicate version and separate it out into sections and bring each piece into zbrush and do your sculpts on individual subtools.

Once you are happy with your sculpts you will need to create your displacement maps or normal maps. If areas of the model share the same uv tiles you will have to combine maps together. Also its advisable to then update the meshes with the meshes that have been sculpted on and usually at a division level of one or two above the base mesh to allow for better displacements at render time

Cheers
Jay