Airport Router DHCP Client Identification
It seems there is no way to force a client ID at the router end, so you have to make do with setting it right on the client.  I discovered that my Apple TV and Playstation has spaces in the names that prevented them from setting the ID properly.  Taking the spaces out made them show up properly.  However, some clients manage to register names correctly with spaces, so I guess it's down to the implementation of the IP stack on the client device rather than a server issue.

It's also worth noting that after updating settings or resetting the routers, it appears to show just MACs until it has time at some point doing a lookup against it to set the ID.  I found no way of working out how long this is, seems to be random.

It's also worth noting that some devices that are able to use Wake On LAN over wifi and that are currently in a sleep state will register at a Physical layer, so the router is able to send the relevant command to wake it, and as such the MAC is shown, but no IP (and hence no active DHCP use to pull the ID) until the device wakes and pulls it's IP/registers it's client ID.

Finally, using an old version (5.6.1) of Airport Utility it is possible to view a DHCP lease list of all clients, not just active wireless connections.  Sadly it doesn't work on Mavericks, but you can bodge it to make it work.  Unfortunately, either it does not track the DHCP leases for the guest network, or the lease length (not configurable) is short so pretty much only actively connected guests are shown in this list.