The simplest script would be
this one which removes the focus from the current active pane. (It doesn't matter what the script is actually for, you just want to goo inside.) This is written in AutoIt, but conversion into AHK (or whatever) would be pretty straightforward as the constants and SendMessage() calls are normal, as long as you understand what it's doing first.
Download the zip and just look at the function _GetActivePane() which follows the simple regex-verification that the active window is actually an x2 window.
The function itself returns the ClassName of the active pane, such that:
$SCRAP = "ATL:BrowserListView1" ; This is the "miniscrap" pane, if visible
$LEFT = "ATL:BrowserListView2"
$RIGHT = "ATL:BrowserListView3"
Afterwards, you can use ControlGetHandle() or something on that classname string which would give you the handle of the actual pane-control itself, which then lets you manipulate it (and thus its contents) any way you want.
Like your own idea, it's based off the Addressbar control, but it's not subject to any real interference, and I consider it 100% reliable, a status which not even my own mum qualifies for.