4.0 ice cream sandwich – Install android 5.x.x on sony xperia tipo (non-unlockable bootloader) using a fake ota update via a dns sinkhole

The sony xperia tipo is stuck on android 4.0.4 and it just so happens that my sony xperia tipo has a non-unlockable bootloader which means I am not able to flash any custom roms to the device. I was wondering if I could install a newer version of android using a fake ota update, when the device trys to check for an update instead of directing it to the legitimate update server the dns directs it to a local server which has a ota update stored on it. As far as I understand I could issue a full update package to the phone to completely rewrite the partitions on the device (https://source.android.com/devices/tech/ota/tools). But I would also need to know the sign keys for the tipo or else android won’t even try to install my fake update. Would a process like this be achieveable?