In our campaign, we have always allowed power attacking when used for a coup de grace. It is a (a) melee attack and (b) an automatic hit.
Same theory goes with the full-round action to line up an melee attack on an inanimate object. It's definitely a melee attack, and deals melee damage, it just doesn't require a roll.
The Core description of the Power Attack feat says "subtract a number from all melee attack rolls and add the same number to all melee damage rolls".
It doesn't say "all melee damage rolls requiring an attack roll".
Archer claims to hae an obscure source that
does... I'd like to see the reference, too
You shouldn't be able to use PA, because it's not a melee attack roll, and the damage is not from melee attack.
My contention is that "automatically deals bite damage"
is melee damage, since the dragon turtle's bite is a melee attack.
To me, it's less grey than, say, constrict... which isn't dealing damage from a melee source, usually.
Rend is another one I'd consider allowing it Power Attack to work on.
-Hyp.