how t orestrict nsupdate to a single A or PTR record ?

[fddi]

Hello, I have a domain called mydomain.org

I would need a way to allow access with nsupdate not to the entire 
domain mydomain.org
but only to specific hosts and specific IP Address do be modified using 
nsupdate.


here is my config

zone "mydomain.org" IN {
         type master;
         allow-query { any; };
         file "mydomain.org.db";
         update-policy {
                 grant mykey. subdomain mydomain.org. A TXT CNAME;
         };
};

but in this way anyone can modify any hosts in the domain.
How can I restrict and allow to modify only specific hosts ?

for example I would like to restrict to modify only host1.mydomain.org 
with a given key.

is it possibile ?

thank you

Rick