Ok, so I admit it. I thought I had the exclude category code all worked out but I ran into a couple of problems. One was anticipated as a possible issue with my RSS feed but the second was completely self-induced and caused by trying too many things simultaneously and not paying close enough attention.
But, first, I will start with one small success. The digest post was not sent to Twitter late last night. I guess by suppressing the post based on it’s category from appearing on the front page that Twitter Tools didn’t notify Twitter of a new post. It’s also possible that since the post came from Twitter that Twitter Tools does not automatically send it back to Twitter, which might put you in an unending loop. I read something about this on the homepage for the plugin but was not entirely clear on it. I also found out that, at least for me, Twitter Tools sends the post at around 11:40 and publishes it at midnight. I was actually still up and watched it happen.