There are more elegant ways to do this, and its a little hard working out which would be best as there isn't much detail regarding your set-up and what you're after.
A simple way would be to simply intercept the click event of the tab and then change the panel's html to whatever you so desire, e.g.
listeners:{
click:function(){
myPanel.html="<iframe src='http://www.google.com'></iframe>";
}
}
You could also include a check for which tab is being clicked then change the called URL as a result...