You can use the ota update even if you have your bl locked. The easy way to do is to download a complete image for cardflash from the Xiaomi site, then put it in the Downloaded_rom (sth similar, I cannot remember the exact name) directory, then you go to the updater page, on the upright corner, choose "manually select the rom" (sth like that, I use the Chinese language display, so i cannot tell exactly), and follow the instructions, in your case, you can choose either stable versions or developer versions, but once you flashed the developer version, there is little chance that you can switch to stable versions without encountering flashing problems.
