I was hoping to create a dragon shaman using this home brew version of it. My question is what feats should I take to optimize it to be a grapple tank? (By grapple tank I mean that I want him to grapple foes that are about to attack allies)
To get it out of the way...
Full Casters Make The Best Grapplers
Wizards and Sorcerers can buff themselves to have huge grapple modifiers, grow tentacles, change into creatures with Improved Grab, change size, add grapple for no reason, and basically be all around awesome. Druids explode into giant bears (that have improved grab) at level 5, and cast spells that make their strength huge for no reason whatsoever. Psions grapple things with their minds - many things, all the time, all at once. Clerics pray to their god and receive a bonus of every kind to every type of thing, and then proceed to grow to huge size and grapple people while surrounded by alternating coruscating auras of awesome.
Barbarians Can Grapple In Antimagic
Between Rage, Prestige Classes that increase size/effective size, the Bear Totem Barbarian's Improved Grab, and the Rune Scarred Berserker prestige class that allows a Barbarian to (eventually) sheath himself in antimagic and still be able to grapple anything but Titan into submission. This is Important because otherwise spells like Freedom of Movement and Dimension Door completely negate the point of grapplers(although it's still unarmed damage done with a touch attack, but not the point).
Dragon Shaman Grappler
Key Abilities: Str, lesser Dex/Con
Dragon Shaman Class Abilities That Help:
Helpful Dips: Monk 1 (Chaos Monk to taste) gives both Unarmed Strike and Improved Grapple, as well as all good saves and Tumble as a class skill. Bear Totem Barbarian (UA) gives Rage 1/day (take Extra Rage as a feat), Fast Movement, and Improved Grab (for that beautiful and wondrous 'ball of town guards carried above your head' multigrapple).
Grappal tank is hard!
Your best bet is to get them to provoke an AoO somehow, which you turn into a grapple. This means Reach, and worse, Natural Reach.
The easiest way to get Natural Reach is via race. Without that, you're looking at Willing Deformity: Tall, a Vile feat from Book of Vile Darkness, or Aberration Blood + Inhuman Reach from Lords of Madness (which notably opens you up for Extend Reach from Savage Species, a total of +10' reach from tentacle-like arms).
In Complete Warrior there is a 'mancatcher' weapon that is a Reach weapon that can grapple at range. With Unarmed Strike or a Natural Weapon to act as your 'close in' weapon, it's ideal for a grappling defender. You still probably want Natural Reach of some kind, though, if only because having a reach weapon doubles it.
Psychic Warrior (Expansion + Extend Reach) or Wizard (Enlarge Person + Fearsome Grapple) are two relatively low-level-requirement ways to get size-increase (and therefore reach increase) or just straight reach increase from class. Psywar has Grip of Iron that boosts grapple checks by a big number, and Wizard can get a familiar (octopus, stormwrack) that gives a bonus on all grapple attempts (hurl it at flying enemies while shouting something about how your burning soul cannot be contained!).
Your best bet is probably Readied actions, however, like a Linebacker counter-tackling someone. There are ways to get to act as an Immediate action, but most of them are high-psion or high-wizard, and well outside your pay-grade as a Dragon Shaman.
With Improved Grab and a big enough modifier, you can even deal with a horde of attackers with a good dex and combat Reflexes. Just snag'em as they go past.
Grappling as AoO: Starting a grapple is an attack action, so can be taken as an aoo. Some people will say that you can't take the 'free action' to make the opposed grapple check after 'grabbing' with the touch attack. This is dumb. If the rules let you make the touch attack, the rules clearly want you to continue the grapple process with the opposed grapple check. This is a clear case of nitpicking, and if people try to seriously make this argument, then you are in deep neckbeard territory and I cannot help you.
Useful resources: Hulkamania runnin' wild, the Grappling Handbook.