It is important to note what is 'regardless'. As precedent, I remind everyone that the old ATSKNF stated something along the lines that marines could always regroup 'regardless' of being under half strength. Then a lot of people began regrouping at all times, even when enemy units were within 6". But we now know that only the half strength part was the 'regardless', and was the only part to be considered when following the rule. I believe that the same thing occured with the older wording for the resurrection orb: it allowed Necrons to always attempt a WBB 'regardless' of the strength of the weapon, but players began using it against power weapon attacks, which was also incorrect.
Following that train of thought, I think preferred enemy will only hit on a 3+ 'regardless' of weapon skill, and so only matters when weapon skill is used to determine what die roll is needed. Since you don't use weapon skill to plant a grenade on a walker, then preferred enemy does nothing.
However, since the Dark Eldar wargear says 'regardless' of all other factors, I would have to say that they can plant haywire grenades on a 3+, but that's just my take on it. But I find it hard to believe if there is not an FAQ for Goblet of Spite, as this would also mean that wyches hit skimmers on a 3+ as well.
- Oaka