Please advice on making external zone visible on inside.

Hello good people!

I have a customer with internal root name-servers. Now they need to be
able to see one of their external DNS zones hosted on their own external
name servers.

To complicate this further they use the same zones on the inside as on the
out side. They have decided to migrate out from dual copies of the
zone files.

Lets say they have zone.com on their inside and zone.com on the out side.
They want to start use foo.zone.com on the out side but also see it from
the inside. To solve this and without having to double administer i have
thoughts about delegate from the inside this sub zone foo.zone.com to
their external name servers. One thing i am scared of is what if for some
reason the internal name-servers could learn the NS pointers from the
external zone.com. If this happens chaos will ensure.

Forwarding is not an option since this is a huge company spread globally
with hundreds of name-servers.

Have anybody done similar things and have some tips or is there other ways
of doing this.