QUOTE (rathmun @ Jul 23 2009, 07:11 AM)
Regarding the ability to exclude enemies, you can have a 'restricted target' option on a spell. While that restriction can't be "allies" it can be "People wearing a charm that looks like this."
I don't know, you are at least stretching the rules here, if not outright breaking them.
The suggestion for restricted targets is voluntary, or a specific race. The list isn't exclusive, but I think the type of target has to fall into the general type. At a minimum, since people with item "x" is so specific and small, it would actually fall into the category of Very Restricted Target.
Very restricted target, however, is designed for caster-only, or a specific (named) individual (It would be interesting to design a death spell to take revenge on a particular mortal enemy). With gamemaster approval, it can be extended to a small group, such as a male trolls, rotordrones, or water spirits (particular metavariants might also fall into this).
The problem, however, as you see in the box just to the left of restricted targets, is that magic is not intelligent. Thus, I don't think magic can distinguish between targets unless there is something particular about their aura or general physical structure (for drones) that enables the spell to distinguish between targets.
It's a clever idea, but I don't think it should be allowed unless the specified items are magical items that bear the astral signature of the caster. Perhaps a new type of "targeting focus" could be developed. If you go through that much trouble, however, it might be better just to get sustaining foci and cast the spell individually on each target.