[Q] Rooting Without Installing a Custom Recovery topic

I have a Verizon Dev Ed unlocked with root and a custom recovery installed on 4.4. A friend of mine has an unlocked Verizon Dev Ed and would like root but doesn't want to flash ROMs or mod her device; she just wants to install SuperSU to get root. I read on another thread that you can boot into a custom recovery without installing it and then install SuperSU from the custom recovery to root.

Rooting Without Installing a Custom Recovery

After reading that thread it seemed to me on a Nexus at least you should be able to do it by placing the custom recovery into the same directory as fastboot and typing:

fastboot boot twrp-2.6.3.1-ghost-4.4.img.

which should boot you into the custom recovery without installing it. I did this and got an error:

booting . . .
Command restricted
FAILED (remote failure)


Then I tried it again with mfastboot and got pretty much the same errort

booting . . .
(bootloader) Command restricted
FAILED (remote failure)



Any thoughts? Is it not possible to root a Moto X without installing a custom recovery? Or is there a problem with the fact that I have already installed TWRP on my device? I was trying to test this for my friend but I am already rooted with TWRP installed as my recovery.