You can ready a free, move, or standard action. That action can be to cast a spell that has one of those casting times. If that spell is also a ranged attack (which can be a ranged touch attack, a type of ranged attack), you can meet the requirements of the feat Deadeye Shot (PH2 78).
The DM may require the trigger for your ready action to be very specific, however, forcing the caster to predetermine target, spell, and maybe even target's square, depending on how strictly he interprets the ready action.
The DM may balk at such a task because the feat Deadeye Shot seems to assume a ranged weapon instead of a spell, but nothing in the feat's text mandates that. A manticore could use the feat with its tail spikes just fine, for instance.
If the DM continues to resist, point him to the Weaponlike Spells section of Complete Arcane, which says, in part, that
Any spell that requires an attack roll and deals damage functions as a weapon in certain respects, whether the spell deals normal hit point damage, nonlethal damage, ability damage, or energy drain. Such spells can threaten critical hits, can be used in sneak attacks, and can be used with favored enemy damage bonuses. You can even use a number
of combat-enhancing feats to... improve the effectiveness of weaponlike spells... (85)