How can I set up Macs with dynamic IP addresses to update a server with a static IP?
You can try and host your own dynamic DNS service or you could use one of several online dynamic DNS companies. 

While I have not implemented it, this MacOSXhints article appears to be a fairly straightforward solution via a FreeDNS account:

http://hints.macworld.com/article.php?story=20110812232611102