I was struggling with this same problem for a few hours tonight and finally got it figured out.
1. Create a blank page in iWeb and set it to be included in the navigation bar.
2. After publishing your site, find the file iWeb creates called "index.html" and open it with a text editor. (I like TextWrangler, but TextEdit should work as well.)
3. The file should be very short. Find the section that looks like this:
<meta http-equiv="refresh" content="0;url= PageName.html" />
4. Replace the portion after "url=" with the full external URL you want the navigation bar link to point to. For example, your final line might look like this:
<meta http-equiv="refresh" content="0;url=http://myblog.blogspot.com" />
5. Select "Save as...", and use this file to replace the blank iWeb page you created for the Navbar. (If you made Blog.html, this will become your new "Blog.html" file. (Don't just save the file, or you'll screw up your index file. We're just using it as a template.)
For an example of this fix in action, here's my site where I did this exact same thing earlier this evening:
www.timelypersuasion.com
(click on the "blog" link in the navbar to see the external redirect)
Hope this helps.
-J