Hi Gawin
In the export dialog you should choose the coordinate system of your project and write a traslation vector that brings your model to "near" (0,0,0). I usually take the integer part of the first GCP coordinates (with the same sign), take a screenshot of the dialog and save as "traslation.jpg" in the same folder as translated obj just in case.
Then go to zbrush and cook the mesh.
When you ar ready to reimport, use the very same coordinate system and vector including the same sign. If Zbrush does not change scale or anything else, I guess you will have the model back in its right world coordinates. Please tell us if works
Good luck