It sounds like a disarm maneuver. I would give the PC an automatic crit if it's successful (because the troll is essentially coup de grace'ing himself, but he's immune to the death effect of a coup so it's just an auto-crit.)
I don't think action economy allows the troll to both hold the weapon in and continue to hit, he would have to choose one or the other.
Holding the weapon in would be resolved as a readied action - the troll readies a disarm for when the PC tries to pull the sword free. As the PC tries to free his weapon, the troll does get an AoO, and then if his readied disarm is successful the sword stays in.
If the troll full-attacked, he can't use any arms to hold the sword inside himself, so the PC gets to pull out his sword without a check (but still provoking.)
Making it so a foe can't use their weapon sounds a lot like a disarm, so I'd use it. That combat maneuver says "Attempting to disarm a foe while unarmed imposes a –4 penalty on the attempt", and "If you successfully disarm your opponent without using a weapon, you may automatically pick up the item dropped", so it's tough to pull off, but the tank has to succeed at a disarm check himself to get it back. To reflect the idea of the troll intentionally injuring itself on the weapon, it might make sense to negate the -4 unarmed penalty in exchange for allowing the tank's usual opportunity attack provoked by the disarm to automatically succeed, although your mileage may vary. That's what I'd do in my game, but even if the disarm fails, the troll would still take the damage from the opportunity attack.
Okay, I got somewhat ninja'd. I'd say that the troll needs at most one hand to keep the weapon in, still leaving it with one claw and a bite, and I'd say he can just leave it hanging there without using hands, like a sword in a sheath, which I guess would make recovering it a 'Steal' combat maneuver. Disarms are generally in place of a single attack, and I don't see much reason to change that, so if the troll was already within reach, it could go disarm/claw/bite
Okay, I got somewhat ninja'd. I'd say that the troll needs at most one hand to keep the weapon in, still leaving it with one claw and a bite, and I'd say he can just leave it hanging there without using hands, like a sword in a sheath, which I guess would make recovering it a 'Steal' combat maneuver. Disarms are generally in place of a single attack, and I don't see much reason to change that, so if the troll was already within reach, it could go disarm/claw/bite
Disarm/claw/bite is fair on the first turn, the reason I was saying he shouldn't be able to full-attack was because I was treating it as a readied disarm. But your way sounds good too - troll full attacks with a disarm maneuver, and "picks up" the sword which is now "sheathed" in the troll. Then the PC has to use a Steal maneuver to get it back. Fits the mechanics and the flavor.
The only thing that's not really covered by the rules is how much damage to do - whether it's an auto-hit, auto-crit, or what. Since the troll is intentionally pulling the sword farther into his body, it sounds to me like it should be an auto-crit.