I tried doing this, but anything I load with "fastboot boot blablabla.img" doesnt boot correctly. The custom recovery is actually running, as i can connect with adb shell and I get a root shell, but it is completely black screen. I tried with both CWM images, twrp, and even the stock recovery.img. they all load to blackscreen when I use "fastboot boot" but they work fine when I do "fastboot flash recovery blablabla.img".
So, if you dont mind navigating the recovery blind, then sure, you can load up a custom CWM, then find your way somehow to the "install zip-> install from sideload" option. Then grab the installer zip for kouch's superuser and run "adb sideload superuser.zip". wait about 30 seconds, and then "adb shell", and finally "reboot".