If you have an unlocked bootloader and a copy of the boot.img file for this device, you should be able to unpack the boot.img, change the
ro.secure system property from "1" (one) to "0" (zero), re-pack the boot.img and flash it back to your device.
That would allow you to then start-up adb (which will now run as root) and install root manually.
There's a couple of caveats and details omitted from the above
*, but this is one way to achieve root.
~ ~ ~
* some of the caveats and details:
- you have to have a copy of the current boot.img file for this device
- you might need to know the boot base address when re-making the boot.img
- you'll need access to a Linux or Cygwin environment to do the un-packing and re-packing
- installing root manually is not really trivial...but Chainfire's UPDATE-SuperSU-v#.##.zip file contains all the files you need and an installation script; however, it does assume that it's being run from a custom recovery
- etc...
Cheers!