A shadow spell remains that shadow spell when that shadow spell is triggered
On Casting Time, in part, says, "You make all pertinent decisions about a spell (range, target, area, effect, version, and so forth) when the spell comes into effect" (Player's Handbook 174). That and so forth, in this DM's opinion, includes for the 5th-level Sor/Wiz spell shadow evocation [illus] (PH 277) and similar spells which spell the spell's effect will duplicate.
Thus, until a shadow evocation spell (and any similar spell) comes into effect, it's still that shadow evocation spell, and if the shadow evocation spell's instead triggered and not cast—as spells are when they're tapped to create magic items—then the shadow evocation spell never comes into effect and whatever spell the caster would've picked to duplicate had the shadow evocation spell come into effect therefore can't be used to duplicate another spell prerequisite but, instead, only the shadow evocation spell itself. (Were this not true, a magic item creator would struggle mightily to create magic items that have as a prerequisite the shadow evocation spell!)
For example, a wizard that's prepared the spell shadow evocation yet not the spell fireball wants to create a wand of fireball by meeting the fireball spell prerequisite of that wand of fireballs with the spell shadow evocation. That wizard can't trigger the spell shadow evocation to meet the fireball spell prerequisite, though, because
The act of working on the wand [and other magic items] triggers the prepared spell, making it unavailable for casting during each day devoted to the wand’s creation. (That is, that spell slot is expended from her currently prepared spells, just as if it had been cast.) (Dungeon Master's Guide 288 et al. and here)
(Note that in the above the slot's expended as if the spell'd been cast and not that the spell's been cast therefore the slot expended!) In other words, until the shadow evocation spell comes into effect, the shadow evocation spell is not another, different spell but the actual spell shadow evocation. This makes the shadow evocation spell (or another similar spell) technically ineligible for meeting magic item spell prerequisites except when a magic item has as its prerequisite the spell shadow evocation (or another similar spell).