That is exactly what I do except the roles of focus and target are reversed. I use the target for poly and focus for dps (or rather, focustarget).

So I go up to a pack of mobs, target a mob, hit a button, and that button sets one of my mages focus to my mains target. Pick another, etc, etc.

Then I open with my polymorph spell and all the mobs get poly'd at the same time.. and I can keep repolying those same mobs throughout the fight.