While you can't equip a mech with a magical suit of armor, the Mech Manual includes Mech Shields & Siege Shields. Siege Shields are an unusual case because they have to be used in pairs to have any effect. This means you need to decide that they opperate 1 of 2 ways: as 2 items for costs and effects (remembering that if 1 is destroyed the other is useless), or treat the pair as a single magical Item for costs and effects (that is +1 would only cost you 1000 thousand gold for both shields, but would only increase AC by 1). As stated in the Mech Manual, both types of shields are rarely used because of the amount of space they need, but with the right enchantments, they are much more worth while. With 1 of these on a smaller mech, combined with a pilot with the Mech Dancer feat, you could potentially get your mech's AC to around 20, instead of in the single digits.
Next we have the various special abilities you can put on a shield. Several of these are very useful on a mech. Animated shields are great as they can be added to almost any mech since they take up no space. Since "A character with an animated shield still takes any penalties associated with shield use, such as armor check penalty, arcane spell failure chance, and nonproficiency" I personally require the appropriate crew is still necessary. Arrow Catching is useful for protecting your crew from being sniped. Since any projectile aimed at a target with 5 ft. (the crew) of the user (the mech), only magical attacks can target your crew. Blinding shields are a great way to discourage boarders. Fortification shields, of the various types, are a great addition to the mechanical mech types, protecting agains crits. An Undead Controlling shield is a useful addition to a necromech, giving them control over an additional 26 HD of undead minions.
The remaining special abilities are fairly situational (energy resist, spell resist) but still useful, but there are 2 that are either unapplicable to a mech or are of reduced utility. Arrow Deflection is of little use, because it is usable only 1/round, can't effect exceptional projectiles, and requires a fairly high DC reflex save, which most mech have a low modifier. A Wilding shield is of course useless on a mech.
You could potentially convert any of the specific magic shields to a mech shield fairly easily. Simply subtract the cost of a masterwork shield of its size from the item cost, then add the cost of an appropriately sized masterwork mech shield. The most useful are the Absorbing Shield, Lion Shield, and the Spined Shield. A Winged Shield is useless as it's weight limits are to low to lift a mech.
Last edited by Raptor on Tue Mar 01, 2011 10:38 am, edited 1 time in total.