I was actually thinking of an actual heat wave akin to this aspect of wall of fire:
"One side of the wall, selected by you, sends forth waves of heat, dealing 2d4 points of fire damage to creatures within 10 feet and 1d4 points of fire damage to those past 10 feet but within 20 feet."
Atop that, add in the heat metal aspect for something like...
Heat Wave (Su): Once every x rounds, a dwarf crusher can emit a wave of volcanic heat. All creatures within x feet suffer xdx points of fire damage to creatures, while those x to x feet from the construct take xdx points of fire damage. All metal equipment carried by creatures within the area is also subject to a heat metal effect (caster level 12th, Will DC X negates). The save DC is Constitution-based.

Oh, and it obviously can't be Constitution based since it's a Construct - switch to Strength or Charisma?

Not true! Check out the iron and stone golem's special abilities.
A lack of a Con score is treated as 10 for purposes of determing save DCs.

That would be way more damage than it does by melee, and it's sort of better than a breath weapon since the wave of damage doesn't get a save. Maybe slightly less than 12d6/6d6? Str-based for the heat metal DC makes some perverse sense.
That said, I could see Cha-based, but I'm not really feeling Str-based for something that doesn't involve it's actual physical prowess.
 Definitely include a Fort save, and I still think 12d6/6d6 is too much since it's a big burst in melee range.  CR 11 dragons don't typically get quite that much.  How about 10d6/5d6, more in line with a CR 11 young adult green dragon?
  Definitely include a Fort save, and I still think 12d6/6d6 is too much since it's a big burst in melee range.  CR 11 dragons don't typically get quite that much.  How about 10d6/5d6, more in line with a CR 11 young adult green dragon?