Do the multipathing first, and the mirroring afterwards!

If you are using a mirrored root under Disksuite and wish to enable mpxio I wish you well. It is a devil of a job, mainly due to the fact that stmsboot won't recognise your metadevice as a STMS device and so won't enable it.

So to fix it here is the answer:

1. Back everything up. EVERYTHING
2. Follow the guide here to undo the mirrored root
3. Run stmsboot -e and reboot immediately
4. Everything should now be correctly multipathed using mpxio
5. Rebuild all the mirrored root and other devices

and next time, do the multipathing first and the mirroring afterwards!