[Plane]

Unless the grappler wants to let go of you as a free action, perhaps whatever he's holding could be treated as a collective higher-mass object as him by totalling up his current ST with the max HP of the object or person? If that's how you'd resolve "I push-kick his shield" then it seems like how you'd resolve this.

That sounds kind of like a disarm so maybe knockback applied to held objects/persons could somehow morph into a disarm or "break free" attack on the wielder.

Of course, wielding heavy weapons doesn't normally give bonuses against knockback when the wielder is hit, despite the added inertia, so I could be wrong in thinking that being wielded gives a weapon more inertia against knockback.

You'd probably need to be strong enough to carry the person grappling you to treat them as your own gear and increase collective knockback. A dragon being grappled by a hobbit shouldn't be unable to Roll With Blow of an incoming hammer, but a hobbit being grappled by a dragon should be penalized to the point where it isn't feasible.

Gun-Fu 11 "Ranged Disarms" introduces an alternative to the usual means of resolving disarms, instead of a Quick Contest of weapon skills (with option for either to sub ST for DX), if the attacker hits and the target doesn't parry/dodge, instead of the QC there is a one-sided ST roll by the target to maintain their grip with -1 per 2 points of damage.

This was designed for guns but I don't see why you couldn't use that option for all disarms, and this would be an easy way to resolve whether or not people maintain their grip on something.

-1 per 2 points of damage also resembles Technical Grappling's control points... coincidence?