Thanks, @kench928 ,it's good to know there is a transformation method! I have European SGP321, but it can help to "debrand" Docomo tablets. The hardware is the same, just the firmware and partition layout different...I can confirm it's work on docomo so-03e (with SGP321_10.4.B.0.569_4.3 flashed).
[ You can check here on how to change your SO-03E to SGP321 with 32GB ]
Started with step10, and just Checking Exclude Partition when doing step12. (12. Flash everything in 'SGP321_4.4.stripped.ftff')
Tested camera, speaker, nfc, wifi, lte. All work.
Radio/televsion service (docomo version only service) should not work.
Risk on your own.