$( "a[class='NetObjectLink'] " )[0].attributes.ip.value
You can also get other values that appear in the node popup.
The reason for using the node popup data, is that it can appear several times on the same screen, in the DOM, but it does not matter which you pick (in the above script I have grabbed the first - element 0).
The NetObjectLink also has other handy values:
$( "a[class='NetObjectLink'] " )[0].attributes.href.value
$( "a[class='NetObjectLink'] " )[0].attributes.nodehostname.value
To use this, put a Custom HTML element on screen, with <SCRIPT> tags, and the script in that.