MX record for a delegated zone in a parent?

[Rongsheng Fang]

Hi List,

We are running BIND 9.3.0 on the name servers (ns1 and ns2) for our
domain (foo.com). And we have a delegated zone called us.foo.com in ns1
and ns2's config. The name server for zone us.foo.com is not in the DMZ
and can only be queried from intranet, which means any records (A, MX,
etc) for us.foo.com cannot be queried by public. But now we need to make
MX record for us.foo.com queryable by public while still keeping all the
other records invisible to public. 

My question is: is this doable? If so, how? (we are not going to put the
name server for us.foo.com in the DMZ unless obsolutely needed).

BTW, I am not subscribed to the list yet. So could you please kindly
copy me when you reply?

Thanks,

Rongsheng Fang