If you edit the blogging.rb file in the blogging bundle support directory (or make a copy in a new bundle) and add:

@post['mt_excerpt'] = @headers['excerpt'] if @headers['excerpt']

around line 224 (it's not especially important, as long as it's in that block) it will send a header in this format:

Excerpt: your text without line breaks

You'll want to edit the language file to include the [Ee]xcerpt header so it highlights properly.

Then you'll want to add 

doc += "Excerpt: #{self.post['mt_excerpt']}\n" if self.post['mt_excerpt'] && (self.post['mt_excerpt'] != '')

at about line 400 in blogging.rb so that it will retrieve the excerpts when you fetch the post or after posting.

That's it.  Excerpts should work for you then.

Brett