Ok, so this might be a bit complicated.
D&D 4th edition might get you in really, really weird situations sometimes and what worries me is not the logic decision behind things but the balance issues.
This has happened more than once, honestly.
A Large monster has a Reach 2 attack power describe as Claw or anything that would make him use a body part to attack. The player has a protective wall/zone/area-of-effect between him and the monster that will cause damage to anyone who enters that square. The monster does not enter the square, but attacks. The character says: "So his arm goes through my wall, right? So the monster takes damage, right?" …right?
Usually I do whatever it takes to avoid that kind of situation, but right now I do want to end their heroic tier with something epic and a Kraken will do it perfectly. The problem is, the Kraken is a gargantuan beast that abuses its reach by being underwater and attacking ships while trying to drown its enemies. If it uses the tentacles to attack from reach 3, it grabs the target. This immediately made think: "The players will want to attack the tentacles". The rules would say it isn't possible because they are not adjacent to the creature. I wanna rule that they can attack the tentacles, but on the other hand, it would make the kraken lose all its Hit Points without being able to use the other things that actually make it a challenge (as in its threatening reach or the wavewrack).
That's the core of the question: How can I make this combat balanced in a logical way that will actually be fun and a good challenge?
Are there any rules to support the idea that the monster can be attacked if grabbing with a body part? I know there isn't supposed to be read out of the book and just ruled by logic because the answer should be obvious, but we all know it's just not how D&D 4e works. And if someone could clear that out, please, I'd like to know how it works by RAW.
(Please, avoid trying to help if you're not aware of D&D 4e balance works. I'm not looking for an answer from D&D 3.5 players like "don't play 4e" or "It's obvious they should just be able to hit the Kraken").
Best Answer
I did a very exciting battle with a Kraken in D&D 4e. Like you, I realised that the players would try to attack the tentacles. But I also realised the Kraken entry in the Monster Manual is kinda bad.
So instead I designed a custom monster. In addition to leeching some powers off the original Kraken, I gave it a "raise tentacles" power that would summon a number of Tentacle minion creatures. Hitting one would "kill" it, causing the Kraken to pull it back and dealing it 5 damage. The Tentacle minions would take up 2 spaces and have no Reach, but they would move around a bit.
This helped me get around the limitation you mentioned and made the battle much more exciting and lively, as the Kraken would slowly withdraw arms and then occasionally spend an action to bring most of them back into the fray. Felt much more like a giant squid trying to crush a ship.
(I also made the beast rise up at the end of the ship and bite at things, so the players could also attack its head if they felt like it, with the risk of being shoved into the thing's mouth)
If you don't want to go the long way or need a quick option for other situations where a large beast with Reach swings out with a large appendage; I allow players to ready an attack on the creature when it swings out, and a player who is grabbed is allowed to make attacks against the creature as long as the grab rules allow it. But I don't trigger "enters the square" effects for them, as it's only a small part of them moving. It doesn't seem to cause any major balance issues. Most people don't really want to be grabbed or mauled by things so big they have that kind of Reach, so they make other plans to deal with them.