Okay, I got this to work by modifying /wp-content/plugins/seo-ultimate/modules/meta/meta-descriptions.php. I modified line 237 to read:
echo "\t<meta name=\"description\" content=\"$desc\" />\n\t<meta property='og:description' content='$desc' />\n";
This adds in the Open Graph description meta tag. Then, I went into my plugin that was adding in the Open Graph code and commented out (//) the description section.
Initially I had tried to go the other way, but I couldn't figure that out. Essentially, I think the problem is that a plugin can't rewrite the excerpt because the_excerpt can't be modified outside of the loop (I think). I'm not a coder by trade, so use at your own risk.
Also, when you update SEO Ultimate, this change will be overwritten and you'll have to add it in by hand again. I would be very surprised if the guys over at SEO Design Solutions don't already have this in the works.