To tell you the truth, we just haven't had to deal with selecting them as targets. We have a typical left hand navigator which has links to views (traditional views, not yet updated to xpage goodness) and the documents linked in the views just open in the same frame and are configured in the form to open as an xpage. I guess the only place we've had to explicitly point to an xpage is in the 'new document' buttons and those have "computed" URLs, but those buttons were using computed URLs before xpages, so it was just a matter of changing the computed string.
Rich