Editing "Twitter Tools" to add default category IF none is set

This is what I want to do:

1. Post a cameraphone image from iPhone to Flickr using Flickr’s email2blog function.
2. Flickr receives photo, updates my Flickr page, then packages it to be sent to my WordPress install.
3. WordPress install receives data from Flickr, including title, layout, description, links, and posts it.
4. WordPress, via Twitter Tools, posts the title as the tweet text, add a shortened link, and prefix it with the post’s category.

Format: “CATEGORY – Title as tweet text. hxxp://shorten.ed”

I have managed to make it work for posts I create myself, but the category bit is not working for posts received from Flickr.

I am getting: ” – Title as tweet text. hxxp://shorten.ed”

Below was what I edited (well, there are more, but this is the main bit).

$categories = apply_filters('tweet_blog_post_category', get_the_category());
$category = $categories[0];
$cats = $category->cat_name;
$tweet->tw_text = sprintf(__($this->tweet_format, 'twitter-tools'), strtoupper($cats.' - '), @html_entity_decode($post->post_title, ENT_COMPAT, 'UTF-8'), $url);

I suspect that the problem lies in the fact that Flickr does not set the category for posts it sends to WordPress. So when it gets posted by Flickr (through XML-RPC), the post does not have a category at all, not even the default one.

Ideally, I want something that is an if-then statement with the filter of “tweet_blog_post_category.” That is “If there is no category set, go back and set category as cat_name.” Am I using add_category?

Any plugin wizards out there?