You assume incorrectly. He was not constructed in any way similar to a PC at all. His Attack bonus and damage roll is the same regardless of weapon used. The problem is that he might not even have a power usable without the Weapon.
So, you need create unarmed powers on the fly, not allow disarming, precalculate unarmed powers for every monster out there, or perhaps create a houserule that glosses over the lack of a weapon.
Just so you understand that this tactic will soon be the ONLY use of the dominate power against any villains with weapons or implements.
Sure - create unarmed powers on the fly seems to be the "right" solution for a DM (to this approach). In exactly the same way as it is the DM's job to adjudicate the difficulty when the player says she wants to leap in the air, grab the chandelier, swing on it over the raging fire below, and jump to the far side of the room, it is the DM's job to adjudicate the effects of the character's actions if they combine elements of the RAW into something that the game designers didn't explicitly cover.
In this case, though, the game designers DID explicitly cover it; there are specific rules for the effects that a weapon has. Proficiency in greataxe adds +2 to hit. Greataxe does d10 damage. So its pretty easy for a DM to "reverse engineer" all of the bonuses, etc., that were
built into the creature, and adjudicate what the creature's unarmed attack powers might look like.
In fact, the Salamander Lancer example seems to provide explicit confirmation of this: the long spear attack is +18 v AC; the tail attack is +16 v AC. The proficiency bonus for using a long spear is +2.
Regarding implements - any power with the Implement keyword is usable without the implement (PHB1, pg 56), so no harm done. In fact, that's a great opportunity for a villain to laugh at the PC's.
As noted in my previous post, giving the monster an unarmed melee basic attack is *simple*, and prevents "disarmed" from equaling "totally useless", while still giving the PC a benefit for a "cool" use of their power. It doesn't even violate RAW, as it could easily fall under the "customizing monsters" chapter of DMG1.
That leaves you
only the question of how to cope with it. "Unarmed = d4" is certainly one approach, but it is also quite plausible for the monster to grab an improvised weapon from somewhere in the room; that would get them a d8 damage die. Finally, there's the option to go with the appropriate "Low damage expression" entry off of DMG1 p 185. Done and move on; the PC gets a benefit without totally nerfing the monster.
Finally, there's the logic by counterexample. Let's look at the Death Knight.
The Lvl 25 Death Knight (Dragonborn paladin)'s Terrifying Smite includes the entry "Requires soulsword".
a.) Why would the designers include the note "Requires soulsword" if there isn't any way to disarm the monster?
Further, in the discussion of a Death Knight's "Soul Sword" (MM1 p 51), "The Death Knight is dazed and weakened while it doesn't have possession of its soul weapon."
b.) Again, why include such an entry if there is no way to disarm the monster?
Finally, if this is the ONLY good tactic my party are getting out of Dominate, they are wasting a very cool power: as noted above, they can move the creature in ways that provoke plenty of Opportunity Attacks (and Run to guarantee CA for said OAs); they can use the creature to attack the creature's allies; they can move the creature into any trap, terrain feature, or ongoing effect .. as somebody else noted, they may be able to manage multiple such by having the enemy Charge through an ongoing effect, provoking OAs, to a worse location on the field of battle where it attacks one of its allies ..
I think intelligent use of Dominate can cost "the opposition" a
lot more HP in one round than simply disarming it is going to do for its DPR.