Nothing that will work automatically, best you can do is a macro to set focus when you CC and switch back to last target.
Code:
#showtooltip
/clearfocus [@focus,dead][@focus,help][mod:alt]
/focus [harm,nodead]
/cast [@focus,exists][]Polymorph
/stopmacro [@focus,noexists]
/targetlasttarget
Will clear focus if focus is dead, friendly or alt is held (override), then focus your current target if they are not dead and enemy, casts polymorph on focus if they exist or target if not.
The stopmacro is there to stop your target jumping around if you never focused anyone and are using the CC on target.
There may be better ways to do it, this is just what I pulled from my ass and it's messy.
I would be more tempting to go for an elegant mouseover.
Code:
#showtooltip
/clearfocus [@focus,dead]
/focus [@mouseover,harm,nodead]
/cast [@focus][] Polymorph
This one will simply focus when you mouseover then cast on your focus. Also means if you are a melee class you won't be reseting swing timers quite as much jumping between targets.
I am presuming that you want to cast CC at the same time.