You can only cast direct spells on targets you can see. *ball spells hit everyone in an area, including targets you might not want to hit. Unless, of coure, you can't see them.

Imagine a mage with cybereyes. A moment before he releases the Manaball into a crowded room big black censor bars pop up over his teammates, blocking them from his view. A moment later, they disappear again. He had previously designated all his teammates in the same manner as you would for a smartgun safe-target system, and used his free action for the round to trigger the software.

A mage without cybereyes could presumably use AR-style contacts, so his sees his targets with normal vision but digital graphics block his view of his teammates.

Is there any reason this wouldn't work? Or is it in use normally and I've missed it?

I would allow it. However, enemies get cover from allies that are blacked out. Not to mention what happens when the enemy learns what your doing.

Righto. This strategy has come up before, and that's pretty much the ups and downs. the AR could protect your allies, but then hackers or sprites or other assorted issues can make your boon a bane.

Of course, if it becomes a problem, you can just turn off AR goggles/glasses and stick to the old ways.

Why do you think mages gesture so much? They are using their hands to block LOS to their team mates.

With respect to hackers and all that, just do what a street sam does to protect his cyber and other wares from hackers - either disable wireless or skinlink.