You can't post from iWeb to Twitter but maybe you should go the other direction. Personally I think the iWeb blog tool is terrible. I set up a blog in posterous.com (you can email new stuff there and it formats it into a nice blog). It will also post to twitter, facebook, linkedin, and so on. Now on your iWeb site, create a blank page, add the RSS feed widget, put in your info, expand it to fill the page, and your posterous feed will fill in on your iweb site.
You can see how I did this at
http://blog.alpinered.com. The RSS feed is also shown at
http://www.alpinered.com/main/blog.html as well as being posted to twitter and facebook.