Alan,
After looking at the log files on our end and seeing your updates come through, I've identified the issue.
The logic in the Apple products for advertising a global dynamic hostname in DynDNS.com has a precondition of determining whether or not a hostname already exists. That's not to say the specified hostname exists; rather, it checks for any hostname in existence.
This is determined by the device doing a reverse DNS lookup of its IP address.
From the perspective of the Apple AirPort device, the IP address is the WAN IP. The Apple AirPort did the equivalent of (where NN.NN.NN.NN is your WAN IP address):
dig -x NN.NN.NN.NN
And found a valid hostname for its WAN IP in reverse DNS. Since the device determined that it already had a valid hostname, there was no need for it register another one at airport-extreme.justtoast.com.
If you look in your Custom DNS zone, you will indeed see your printer advertised, and the SRV records are pointing to this hostname that the Apple AirPort found by doing a reverse DNS lookup. The device only talked to the servers at DynDNS.com when it needed to register the service; it had no need to create another global dynamic hostname, since it had already found one.
Compare and contrast this to your computer accessing the Internet from behind the Apple AirPort. This computer would have an internal NAT IP. When your computer did a reverse DNS lookup, no result would have been returned, since the address is private to your network and you likely did not configure reverse DNS for your network. Therefore, to be uniquely identified, your computer registered a dynamic global hostname at DynDNS.com. Had your computer been directly connected to your cable/DSL model or other means of connectivity (and not behind the Apple AirPort home gateway), it would have done the same thing that the AirPort did: used the hostname returned by the reverse DNS lookup.
It's important to note that all of this is transparent to your users for the purpose of sharing services. They never have to enter "airport-extreme.justtoast.com", they only have to browse "justtoast.com" for advertised services, and their computers will be able to access those services using friendly instance names.
Regardless of whether the actual hosts use the already provided hostnames as determined by reverse DNS or register new hostnames within your zone, the PTR, SRV, and TXT records required for Wide Area Bonjour are still setup correctly, and users can successfully discover the services.
