[3.0/3.5] How to handle a grapple check coming from an inanimate object

[ShadeBlade0]

The Stronghold Builder's Guide (3.0 material, but allowed for our 3.5 campaign) has a section about designing custom traps. Melee traps are said to "deal the damage of the weapons they 'wield,'" and there is no given limitations on what melee weapons are allowed. The Pincer Staff (Underdark) has the effect, "if you hit an opponent...you may attempt to start a grapple as a free action without taking attacks of opportunity. If you get a hold, the staff grabs the opponent and deals its normal damage each round the hold is maintained."
So this hypothetical bear trap would be able to attack an opponent who activates it, and then this inanimate trap needs to make a grapple check. This trap has an attack bonus, mighty damage (which if you squint could be seen as a strength modifier), and a disable device DC.
What bonus should the trap get for grappling?

[Dimers]

Compare it to bigby's grasping hand and black tentacles in the PHB, or stony grasp and earthen grasp in the Spell Compendium. Those have modifiers built-in. Then just ask yourself, should this trap be better or worse at grappling than these other statted non-creatures?

[ShadeBlade0]

The people I play with are sticklers for rules. Arbitrarily setting a Grapple bonus that is balanced will feel dirtier to them than either having the grapple bonus be the trap’s attack bonus or say that the trap grapples until it is disabled.

[Mr Adventurer]

I wouldn't do it this way - I'd assign an escape DC for Str or Escape Artist. The difficulty you are having is that you are using a weapon to model a bear trap, then worrying that it's not clear how to make it do what you want. In effect, this isn't a bear trap - it's a complicated mechanical trap that sticks a pincer staff out of a hole on an articulated arm and moves about to try and keep the target pinned in place.

But if you insist, I'd say to use the trap attack bonus (since it's BAB+Str).

If you say it's a bigger than normal weapon, treat it as being wielded by a larger creature too, for size modifiers.