Just found this from a post on the official forums.
Auto Party Buttons
This will only work in LFD dungeons but if what I gather is correct, that's what you are looking for anyway.
From the addon description it says it does exactly what you are asking for.
Code:
Currently Implemented Magic Buttons
All Classes:
* Set Focus to LFD Tank
* Assist LFD Tank
Hunters:
* MD to LFD Tank / Raid Tanks
Priest:
* Self Bubble Mostly used for testing
* PW:S to LFD Tank / Raid Tanks Mostly used for testing
* Pain Suppression to LFD Tank
* Fear Ward to LFD Tank / RaidTanks
Paladin:
* Divine Intervention on LFD Healer
* Hand of Protection to LFD Healer
* Righteous Defense on LFD Healer
* Sacred Shield on LFD Tank
* Beacon of Light on LFD Tank
Druid:
* Innervate on LFD Healer
* Rebirth on LFD Tank & Healer
Mage:
* Focus Magic to LFD Healer
Rogue:
* Tricks of the Trade on LFD Tank
Warlock:
* Devour Magic on LFD Tank
Warrior:
* Intervene on LFD Healer
* Vigilance on LFD Healer
Shaman:
* Earth Sheild on LFD Tank