Tatsuo,
regarding more details, I am not sure what I can add to my earlier response. The way its produced is like this: SHA-256( sector_identifier | resource_owner | salt ) (where salt is a value generated with different values), which is how it was specified originally.
For your colleague, there is no reason for him to change the way sub is generated. If he still wants to do so, he has to make sure it is done the same way in future versions, he has to maintain it himself. And we are not able to help you with issues that may occur by doing so.
Regarding sub in general, would it be helpful for you if you could provide an already existing value rather than have OTK generate it for you? This would be good to know for us to adjust future versions.
Thanks,
Sascha