iPhone LTE data with a Docomo nano SIM

Via commenter Willy comes a post purporting to have LTE data working on an iPhone using Docomo's Xi network. The post is in Japanese, so I'll explain below in English. It seems he:

1. Started a contract for Xi with a Docomo nanoSiM (aka nanoUIM) with an factory unlocked iPhone.
2. Turned on data roaming (Not sure why - maybe because DCM is not in carrier.plist?).
3. Set the APN to open.mopera.net.
4. Used iTunes to restore the 6.0.1 firmware, downloadable from here:
   http://appldnld.apple.com/iOS6/041-8531.20121101.Fvjr6/iPhone5,2_6.0.1_10A525_Restore.ipsw

His carrier version is 13.0. He confirmed that power cycling the phone, yanking (and replacing) the SIM, and turning off roaming didn't affect the ability to connect to LTE. Turning off automatic carrier selection shuts off data, however.

If this works for a Docomo SIM, it should work for a b-mobile SIM. I don't have any idea why simply restoring the firmware would work. As always, do any of this sort of thing at your own risk!