Let's have a go at what I believe is the most tank-hostile BG of them all.
Basically, we have nothing to offer here, or almost nothing.
The battle is split into two rounds. One attacking and one defending. Attackers protect siege engines, pick up bombs and commit general mayhem to gates in a wild race to the last gate. They need to remove slowing debuffs from the engines, slow defenders, capture graveyards and get to the final goal in short time.
Defenders, well, they have to make this not happen. So, slow engines, slow attackers, defend graveyards (within reason) and overall become a major PITA at all natural choke points.
We have poor snaring abilities, a stun on a long CD, are suspectible to CC, do abysmal damage at close range and more or less no damage at all at long.
As an attacker, try getting up on an engine. The driver doesn't use any abilities anyway. Just drive and destroy. When your engine is about to blow up, desert it and get up on your mount. Ride the hell away if possible and pick up a new engine.
Later, always pick up bombs before picking up your engine. Same rule as above for almost destroyed engines, but this time you drop at the gate and deploy the bomb. Now our much maligned Consecration shines. People taking damage don't defuse bombs. Stay on top of the bomb and suicide while using every AoE attack in your arsenal. Anything that makes that bomb go off is a bonus.
If you're at the relic chamber when the gate breaks, and if you have the ability available, bubble immediately and rush for the relic.
Don't bother getting down to the dirty job. We have little to offer. Go grab a cannon instead. Bomb away at anything that moves, especially engines. When the gate you defend goes down, jump down on the enemy side of the gate. Take the portal to get back home faster.
We're doing poor damage, but we're still good at AoE tanking. Tank the graveyards. People taking damage can't capture any graveyards, and you can survive longer than a squishy. Suicide time again.
At the last choke point, and definitely at the Relic Chamber, please do bubble your healers. You're on defusing duty if the attackers allow you. Doing poor damage, remember?
So, are we entirely worthless? Well, we share a small benefit together with Frost DK. When a gate breaks down, ride at +20% speed and capture the graveyard associated with the gate that is still intact. Chances are you'll be alone capturing it, and you'll get a head start at picking up bombs and start driving for an undefended gate. It's not much, but it's something.