Further debugging through the logs
Comparing the start up of this MDS/MGS pair with a know good system,
On the known good system:
mgc_copy_llog()
lustre_start_simple() starting obd goodfs-MDT0000-lwp-MDT0000 (type=lwp)
..etc etc
On the bad system:
mgc_copy_llog()
/* no lwp stuff at all */
For some reason, my MDS/MGS pair are not invoking the lwp OBD.