I found a pretty interesting hack to get your ReadyNAS to serve as a dynamic DNS client.
I have not tried it out as it looks like you can only schedule it to run up to every 4 hours, and I'm not ready to commit to a 4 hour outage if my IP changes at the wrong time. But, if that were the last service I needed before I shut down one of my servers I'd jump on it.
In theory the next version (4.0) of the ReadyNAS software will open up SSH/Admin access, which would allow you to do just about anything. I'll hold out hope for that for now.