I suppose I could try it myself, but I'm away from the server.
Can you have something like:

zone "20.10.in-addr.arpa" in {
type master;
file "master/20.10.in-addr.arpa";

and also have.. in the same master's named.conf

zone "10.in-addr.arpa" in {
type forward;
forwarders {; };

Where all 10's except 10.20 would be forwarded?

I have basically a server that I just have the
ability to forward to that claims to be authoritative
for all of 10.... but I want our master to be
authoritative for 10.20 and forward the rest.