Can it be done?

[JungleBox90]

Is it possible for a character to use a non-melee oriented supernatural ability (e.g. breath weapon) on an attack of opportunity?
If so, what feats/spells/items/etc would be involved to make this possible?

[Person_Man]

There are a few feats which replace an Attack of Opportunity with other mundane things, like Evasive Reflexes (5 ft step) or Hindering Opportunist (Aid Another).

The closest thing I can think of that you might be looking for would be the Opportunity Power feat from the Expanded Psionics Handbook. You must expend your psionic focus, and can replace an AoO with a Power that you know with a range of touch whose manifesting time is less than 1 full-round action. You can then manifest the Power as an Immediate Action, and doing so increases the power point cost by 6 (and is thus limited by your overall manifester level).

There's also the Epic Spell Opportunity feat, which lets you replace an AoO with a touch spell.

You can also deliver spells and other effects via melee attacks, such as a Spell Storing weapon.

[TroubleBrewing]

Given that there's a precedent with Opportunity Power, I don't see why you couldn't make a metamagic feat like "Opportunity Spell" with the same requirements (1/round only, touch spells only, +3 to the level of the spell you want to cast).

EDIT: Well, Spell-storing weapons work way, way better for this.

But homebrew is sometimes an option, so my suggestion stays.

[CTrees]

In the given example of breath weapons, i'd see no issue with "opportunistic breath" or what have you, as a new feat.