I got Tomato firmware for my WRT54GL router. It's 5 years old, but tomato does not have new update. As such it doesn't know about dynu.com as a DDNS provider.
I've looked into Custom, but their Custom setting only asks for a URL. It does not list username and login etc.
Is there a URL I can use to update my IP? I understand we can't transfer the username/password in plain sight. Perhaps some form of token?

Yes, you may use the following URL for IP update for Tomato firmware:
http://api.dynu.com/nic/update?hostname=example.dynu.com&password=PASSWORD

For more information, you may refer to setup page for Tomato firmware .

Thanks, but I can't follow your Tomata setup link. It points to a the same link as your example URL

The correct link is http://www.dynu.com/DynamicDNS/IPUpdateClient/Tomato-USB.

I apologize for the inconvenience.