So on the livestream, Steve was showing off locking a reference image to the symmetry plane and grabbing proportions from there and that's sick but I thought it'd be possible to take it one step further and sort of "trace in" using the reference as a template for a bas relief.
So it takes a bit of mucking about but I found I can block it in pretty quickly, trying to work inward from the highpoints back, ie the nose then the brows and so on. This way I'm not obscuring any more of the reference than I need to.
For detailing, because I can't see the picture any more, I turn off the lock and I can pull the sculpt through from the back of the reference image like in this clip