> This will only work if tinydns supports zone transfers (AXFR/IXFR), which as far as I can tell, it doesn't.

http://cr.yp.to/djbdns/axfrdns.html

It does. I'm doing this currently, I host primary DNS myself with tinydns, and the makefile sends a notify to the registrar-hosted slave DNS, which gets data via this daemon (with tcp server configured to only allow that IP).