Miscellaneous Features on Slender Pillar says
These pillars are only a foot or two across, so they don’t occupy a whole square. Place a dot in the center of each square that has a slender pillar in it, and don’t worry about exactly how much space it takes up. A creature standing in the same square as a slender pillar gains a +2 cover bonus to Armor Class and a +1 cover bonus on Reflex saves (these bonuses don’t stack with cover bonuses from other sources). The presence of a slender pillar does not otherwise affect a creature’s fighting space, because it’s assumed that the creature is using the pillar to its advantage when it can. A typical slender pillar has AC 4, hardness 8, and 250 hit points. (Dungeon Master's Guide 64)
Does a creature sharing a square with and using to its advantage a slender pillar only gain the listed bonuses or does the creature also gain the additional benefits of cover? Specifically, is such a creature immune to attacks of opportunity because a foe "can’t execute an attack of opportunity against [a creature] with cover relative to [the foe]" (PH 151)?
Background: While looking for inexpensive ways for monsters to protect themselves from a particularly vicious tripping PC (that is, the PC really trips foes; that PC isn't, like, on drugs), I saw the 1st-level Drd spell climbing tree [conj] (Complete Mage 99) which, for 1 min./level in an adjacent square of soft terrain (e.g. dirt, sand, soil), creates an up to 10 ft. tall/caster level tree (hardness 5, hp 120) that can be climbed with a Climb skill check (DC 5) but can’t be animated nor be used in conjunction with the 5th-level Drd spell tree stride [conj] (PH 296). The climbing tree is otherwise like a slender pillar (DMG 64). (For this reason spells seemed a more appropriate tag than, for example, dungeon-design or terrain.)
My hope is that the spell climbing tree is cover enough to prevent attacks of opportunity, but the phrasing of the slender pillar rules is imprecise enough that I can't be certain.
Official rules preferred, but, in their absence, house rules are acceptable.