Not sure if it is somewhere in the xml files but I do know you can get them to chase rocks and snowballs (as long as they are not aggroed on you already)
Edit:
I just checked and line 15633 in the items.xml does have this line.
<property name="ThrowableDecoy" value="true"/>
maybe that is one that makes them chase it.
It is also line 15665 in items.xml for the snowball.
Could try that in your mod.
Edit of Edit.
Ok never properly read your post. I saw you got them to chase it.
As for them chasing it instead of you after they are already chasing you, I am not sure about.
Am going to leave what I just said though in case someone else needs to comment on it.
Hope you get it working and I see it out there.