On further thought, I think this might be a cool Legacy item. Maybe a Breastplate that is a combined Legacy Armor / Wondrous Item. So Armor benefits, and then has spell resistance imbued into it.
Maybe: Lvl 2 – Protection from unerring spells- Wearer always gets a savings throw VS ANY spell. Will for mind spells, Reflex for attack spells, Fort for disease and poison.
Lvl 4 – Spell Resistance 2
Lvl 6 – Shield spell-like ability.
Lvl 8 – Spell Resistance 10