Go ahead and use your guisarme with Defensive Throw
On Making a Trip Attack says, in part, "Make an unarmed melee touch attack against your target" (Player's Handbook 158). Further, on Tripping with a Weapon, in part, says, "Some weapons can be used to make trip attacks[, and if you do] you make a melee touch attack with the weapon instead of an unarmed melee touch attack…" (159).
While the Defensive Throw feat's prerequisite of the feat Improved Unarmed Strike hints that the feat's author may have wanted the feat to be used with unarmed strikes exclusively, this reader can't find anything in the Defensive Throw feat's text that changes how a trip attack works, and trip attacks can be made with appropriate weapons like a guisarme. Even the feat's description is vague enough, saying, "You can use your opponent’s weight, strength, and momentum against her, deflecting her attack and throwing her to the ground" (Complete Warrior 97), not mentioning exactly what kind of weapon knocks down the foe.
However, as the feat's benefit changes only the circumstances in which this specific trip attack can be performed, this DM would limit a warrior that possesses the feat Defensive Throw to making trip attempts only against a foe within the warrior's reach or within the reach of the warrior's trip-capable weapon. For example, this DM would not allow a warrior who picked as the target of his Dodge feat an archer on a hill a mile away to make against that archer a trip attack with a normal manufactured weapon or unarmed attack were that archer to roll a 1 on her attack roll against the warrior.