I don't see a problem with nuking someone's worn clothes/armor. If it is security or military grade armor, you need to get past the rather high OR. If you are wanting to use this trick in combat to soften up hard targets for the other runners, it is often easier to just powerbolt the wearer, likely killing him outright. The exception would be if the wearer was under spell defence/shielding and the GM ruled that such defence did not extend to what they were wearing.
Assuming no spell protection, and a mage trying to soften up a target wearing heavy security grade armor (OR 9-10 in my opinion) the mage would need to sling a force 5 powerbolt. If he wanted to be certain it would go down in one hit, he'd need to cast it at Deadly (Or Serious if he was willing to do a karma reroll). Assuming he puts his all into the casting (Say Sorcery 6 + 6 Magic pool - Yeah, experienced runners could toss alot more), he's likely to get that one success at TN 9 needed to make the armor go away.
On the other hand, lets say he just cast at the hapless goomba wearing it instead. We'll assume that this guy is a combat hardened veteran (who forgot to geek the mage first). Lets set his body at a 5. Same spell, same force. The mage will likely get 4 success, while the target gets 1 or 2 on his resistance. Odds are, the mage will be able to stage the damage up one level. End result is that the mage can cast the spell at a lower damage code, thereby lessening the drain. Furthermore, the target is now eliminated as opposed to softened.
But you get more style points for blowing up his armor.
I suppose that if a GM were really concerned about abuse an arguement could be made that the wearer got to make a resistance roll for the item, claiming it was within thier aura.
Any book-ninja around that knows if a individual's aura extends past thick security armor?
As far as barrier rating and spells, I don't know offhand.