Creating blocks would be no problem.
Justifying a Weapons Defense roll with breath Weapon... that i would not allow.
Breath weapon basically uses the Thrown Weapons trapping. And you wouldn't allow someone to defend with a shuriken for example, would you ? I wouldn't.
You can however easily construct a stunt that transplants the Dodge trapping to weapons, and then describe your defense as a use of your breath weapon.
Addendum Incorporating the latest post:
A throwing axe could however just as easily be used as a melee weapon as well as a thrown weapon, so there is easy justification for a melee defense roll. Breath Weapon does not necessarily provide that justification.
And not every possible thrown weapon, take the shuriken example i mentioned, provides that justification either.
Also: carrying a throwing axe does not provide the same benefit. A throwing axe can be used once, then it has to be recovered. It can be taken away. Breath weapon removes those limitations.
So, i'd simply judge Breath Weapon to be a thrown weapon of which you have an unlimited supply, which is great, and that cannot be taken away from you, which is even better, but does not provide justification for a melee defense roll.
You can however construct a breath weapon that would provide said justification, but i think it's pretty balanced even without that.