Hiya,
Since I got no responses here, asked support and got this useful reply. Pasting here for anyone else's reference:
Thank you for contacting us.
You can use the console to change the position of the model and cameras relative to the chunk. Sample script:
ck = Metashape.app.document.chunk
m_main = Metashape.Matrix([[1, 0, 0, 0], [0, -1, 0, 0], [0, 0, -1, 0], [0, 0, 0, 1]])
m_cam = ck.cameras[0].transform
m_t = m_cam.inv() * m_main
ck.cameras[0].transform = m_t * m_cam
ck.model.transform(m_t)
#export Depth map
#Further return of the model and the camera to their places
ck.model.transform(m_t.inv())
ck.cameras[0].transform = m_cam