Usually with Orks the Ld value can be substituted with Mob size per the 'Mob Rule!' which is present on page 31. This works with the majority of Ld tests including a Wyrdboy/Warphead, as described in the FAQ
HOWEVER one must look at how the issue of Leadership is addressed. The 'Mob Rule' is specifically addressing any/all Ld checks that an "Ork Mob" might be called upon to make. In this way we have to see the distinction between Unit and Individual. The only Individual who is granted any luxury on the part of these Ld tests is the Wyrdboy/Warphead, and that is only due to that being clarified in the Errata of the FAQ (and also by the fluff, but fluff rarely figures into gameplay at the current date).
This being the case any shooting attack or Psychic attack that targets a single individual and forces them to make an Ld test is being considered an individual and therefore NOT as a unit. In strictest legalese the model targeted would have a Mob Rule rating of 1, as the area of effect is only on that sole individual.
That being said Psychic powers such as an Eldar Farseer's Mind War or a Tyranid Broodlord's Hypnotic Gaze supersede or overwrite the protection of your Nobz (or any other target) through Mob Size.
While there isn't any way to get around the targetability of a Farseer (save range), there is a way to get around the Broodlord's power. Simply don't allow your Nob (or Warboss, or any other Burna or Pk wearer) to get in Base Contact with the Broodlord, as the rules for Hypnotic Gaze (pg 62 of the new Tyranid Codex) state that his range is strictly limited to Base Contact.
Sadly unlike the Farseer a Broodlord will always have this Psychic Power at his disposal, where most Farseer's (at least that I've played against recently) are taking Fortune and Doom (and occasionally Guide) over Mind War. The benefit though is that most Nid players I've gone up against don't take Broodlords since the new codex as they no longer fill an HQ and cost as much as 3 Genestealers or a single kitted out Warrior.
I believe this issue to be QED (Quod Erat Demonstrandum) in favor of the Nids and Eldar.