Thanks for the tip rmn, you are the man!
With opennew it tries to bookmark about:blank (it seems that document.location and document.title correspond to the new page being opened), but open works just fine.
Note that the same results (opening Bookmark4U in the same layer) can be accomplished avoiding the "open" in a bookmark: javascript:location.href='
http://site.address... But now that the macro is written, I'll get rid of the bookmarks plugin.