I'm not using Message Central, but I have seen my Chromecast queue get backed up as well.
I posted about it here and was referred to your thread by @rcjordan. In my case, I just hit refresh on my Chromecast group that I speak to and the queue (annoyingly) flushed out. I'm not sure what caused the queue to start holding messages though.
#HubVersion=2.0.4
