RapidWeaver’s approach of links in new windows is to give the link a rel attribute and a value of external. That’s what happens when you check the ‘link opens in new window’ option. But it’s dependable on Javascript to make the link open in a new window, and so doesn’t always work (it tends to fall foul of popup blockers in web browsers or themes that lack the necessary code).
Unfortunately RapidWeaver will not let you add target as a custom attribute (it throws an error saying the attribute is already in use), so there is not much else you can do in the link dialogue window to force links into new windows. The target=“_blank” approach is mostly only good for links you manually setup as HTML markup, as discussed above.