I looked for a solution to this problem, and came across the present page. I implemented the above solution:
$("#testdiv",opener.document) //doesn't work
But it doesn't work. Maybe it did work in previous jQuery versions, but it doesn't seem to work now.
I found this working solution on another stackoverflow page: how to access parent window object using jquery?
From which I got this working solution:
window.opener.$("#testdiv") //This works.