After looking into it more deeply it seems this was actually symptomatic of a deeper problem - namely the way our Toolbars actually work under the hood.
As of 3.2 and earlier, a special Toolbar Layout creates a very deeply nested DOM tree and injects Toolbar items into it at certain positions. While looking in to this bug I refactored this class but realised it should really just be an HBox layout.
I've rewritten a lot of this code now to use HBox everywhere (ditto for TabPanel), which allows us to remove a bunch of code and gives us performance benefits. It will also make this reordering behavior built in and fully functional.
Although I have it working well locally, these changes are brand new so won't make it in to 3.2. They will be present in svn very shortly and we'll thoroughly test them before they make it into a release.