I vaguely remember something about this from 2nd ed forgotten realms. Has anyone seen this in 3.5?
Spell Mantles were in the Secrets of the Magister, if I remember correctly. While Spell Mantles specifically have not be reworked into 3.5, in Unapproachable East there is a "magic item" called a contingent spell, which is a spell that is crafted as an item and "attatched" to a person, which is set to go off under a certain circumstance. Since it is a crafted item, it doesn't take up any spell slots, and there is a limit to how many one can have crafted for themselves and carry at one time, but normally you can see the spells that are "attatched" to the person.
When they describe the contingent spells they mention that simiilar magics have been found and researched by various societies over time, which I took as meaning that these are fairly similar to spell mantles.
I actually was thinking about using these rules to expand into spell mantles, which essentially would work almost just like contingent spells, except that the person to whom the spells are attatched could activate them at will without the conditions being met, or that they could willfully supress the trigger of the contingent spell if they wanted to, but I would have to think about it a bit more before I was happy with the idea.
At any rate, check out Unapproachable East for contingent spells.