It doesn't seem this is going to happen anytime soon! So I wrote a python script to read all .xmp files in a directory, and output a batch file that uses ImageMagick to crop the face regions found from the images and place the result (as a jpg) in a subdirectory 'faces' of the original directory. I can just import these 'faces' subdirectories back into Lightroom and use the print module to print them as I wish.
Although there are some minor traps (eg working out how to convert between LR coordinates and IM coordinates, handling rotations( I doubt I do but minor rotations are OK), etc it was a relatively trivial exercise and I can't see why Adobe can't provide such a feature. In fact it would seem to be trivial to add a "Export Faces" checkbox in the Export dialog.