Like most SIM-locked iPhones, the Docomo variants do not allow for setting mobile access point names (APN). Without this ability, even if the phone was unlocked to allow usage on other carriers, you would be unable to connect to the data network (except with a particular carrier in the Philippines that completely ignores APN settings).
[List of NTT Docomo APNs.]
This means that you can only use the "sp-mode" ISP from Docomo. You cannot use the "Mopera U" ISP, also from Docomo.
Conversely, to use any Android phone with one of the large numbers of MVNOs does NOT even require paying the ¥3,000+tax fee to have the phone unlocked, since the MVNOs are using Docomo's network. All you have to do is follow the simple instructions to set the appropriate APN. To date, I have never heard of an Android phone that prevents changing mobile network settings.
Docomo carrier bundle
I spent the morning looking around the NTT Docomo iPhone carrier bundle (
Docomo_jp.bundle; v. 15.1.0). Coming from an Android point of view, it's really interesting to see all the effort put into preventing even the slightest changes. Everything is signed. Not just individual files, but some entries within files are signed, including the APNs.
carrier.plistfile are three APN entries, two of which are blank and disabled. The other one is the standard spmode.ne.jp APN, which (also unlike Android) is being used for both data and tethering. (Turns out we were all wrong on our speculation why docomo performs a forced APN switch when activating tethering on Android).
Also within the bundle are several override preference files (
overrides_N51_N53.plist), which contains an entry that appears to only allow the phone to attach to the spmode APN, and both this file and this entry are signed. So I think that, even if you succeeded in somehow setting a different APN, you'd still not be able to connect.