Alaztech, I looked for a loophole like this, but couldn't find one. Under the rules for normal shooting it's pretty clear that you choose a target within or partially within your forward arc, that's also in line of sight. Then the war machine rule triggers to pivot after that. Then blast away.
I play Dark Elves, all I get is bolt throwers (which are over costed and lost a wound in 8th), so no big sweat to me. But something I'll certainly make sure to point out to dwarf and Empire opponents!