Quick answer is: No. At least not in the form you are attempting. As you said, no matter where you put it there's a problem (like unable to trigger /cast steady shot).

However, you can try a modifier (I know, its not an automatic kill shot with 1 button, but still worth considering). When you think the mob's health is low enough (or can tell with your UI), you can hold shift, alt, or ctrl to use kill shot, then let go of the modifier to keep normal rotation. I have no idea if breaking rotation is worth it... or for any class with low-health abilities. Might be something for the wow forums, unless other members here know.

#showtooltip
/cast [mod] kill shot
/click button1
/click button2
/click button3
/cast steady shot