Upon a quick reading of the codex and
BRB, I don't see why not.
When you fire the first Griffon shot in the salvo, it says you may choose to reroll the scatter
dice (meaning you must reroll both the scatter die and the
2D6) for the initial placement of the template.
For each subsequent shot from additional Griffons it says you may also reroll that scatter dice (although only the scatter die this time, as you don't roll the
2D6 anyway) if you choose to and place the barrage accordingly.
Seems like that's what it's designed to do. Each mortar has a chance to improve their own accuracy, not just the first.
Now if you want to be really nasty, put a Griffon in with a pair of Collossi as a range-finder. Watch those
MEQ go squish!
DoW