Create custom menu items with links relative to the root url of the current wordpress installation.
Simply add a custom link as normal with the url prefixed by %ROOT%. e.g. to get http://mydomain.com/wp/myCustomPage if your wordpress lives at http://mydomain.com/wp then you just write %ROOT%/myCustomPage for your link url.
Create custom menu items with links relative to the wordpress root url.
Not tested with multisite installations.