I have been collaborating with Andrew Shell on the WordPress.com rssCloud issue. Andrew added a note to my support forum post that WordPress.com and the RSS Cloud plugin only accept aggregators running on port 80, 443, and 8080. I have confirmed that the RSS Cloud plugin will accept a registration from an app running on ports 443 and 8080 and provide the correct rssCloud response when the source feed updates. Andrew will be contacting Joseph Scott (plugin developer) at Automattic with this information.