you USED to be able to do this with the S1 qUnlockTool bootloader unlocker. you would pay $20 or so for a credit from one of the folks selling server time, run it, and it would work its magic. i did it to four docomo-branded xperia devices.
however, the places i bought the credits from don’t sell them anymore, apparently, so maybe it is not possible
edit: this is the thread that i started out from https://xdaforums.com/t/closed-bootloader-unlock-allowed-no-to-yes-is-now-finally-possible.4044091/ way back in the distant past