You could try creating a top-level landing page (aka div) with id
#landing and then a separate page/div with id #home. If #landing is the
iUI root, but you link to
then you'll always go to the #home
page (with a visible transition that you might not want), but the
#landing page will be accounted for in the back button. You could then
listen on one of the iUI extension events to call your external http
link and return to your native UI.
The "focus" event for #landing might be the event you want to use -- if
you want to catch things as early as possible (before the backward
transition from #home to #landing begins)
Remi's suggestion is good, too. It will require some DOM manipulation
on your part, but will give you more control.
You might end up having to hack on iui.js a little yourself -- which
the basics working and there's away for us to modify iUI to better
support this type of thing - let us know.