If DynDNS free is a feature you must have on the router, then the Time Capsule doesn't have a way of providing that exact service.
I've found the client that you can install on a computer to work well enough though, and eventually migrated it off my Mac onto my ReadyNAS. I use DynDNS purely for backups of this server, so that the server can use an SSH tunnel back into my NAS to drop off the database dumps every night.
The path that the Time Capsule can support is Wide Area Bonjour. This is what Apple automatically sets up with the Back to my Mac feature of MobileMe, and manual methods do exist.
DynDNS does offer support for WAB, but only on their paid plans.
Being that they are offering this, it may indicate the non Apple pieces have finally solidified enough for production use. I did attempt to get this working about two years ago without much success then, due to some flakiness in the piece needed to update BIND DNS files. I'll have to look into this more now, and see if I can get it working against my domain.