What about having a key mapped to THIS on your pally:

/focus mouseover

and mapped to THIS on your priest:
/cast [target=focusfocus,help] Renew

Set the pally as your focus and make sure the pally's focus isn't really important.

That way, the pally keeps his target (which is what you want). Of course you'll have to bind a lot of keys.

Let me know what you think.