Hello Andy,
in order to modify the reconstruction volume, you should use chunk.region (defined by region.center, region.size vectors and region.rot matrix). Note that it is defined in the internal chunk's coordinate system.
As for BBox - it is used for buildDem and buildOrthomosaic procedures and for export operations.
Unfortunately, there's no direct way to access dense cloud extents. As a workaround you can export dense cloud, import it to the empty chunk and call for resetRegion command, then you would need to re-calculate region parameters from one chunk to another.