To be honest, I'm not sure the functionality your looking for is possible (limitation in the Blizzard API). I can think of a few ways to kind-of-sort-of do it but you would need to code an addon in order to do what I'm thinking.
The closest thing your going to get is the following macro:
Code:
#showtooltip
/cast [target=targettarget] Taunt
Basically what this macro does is taunt your target's target. For example, if a Mage was pulling aggro and happened to be targeting the mob that he/she pulled, then it would taunt that mob. However if the Mage isn't targeting the mob he pulled, then the Macro breaks.
My other idea is to re-roll pally. Righteous Defense ftw.