Referrals don't work like that. Read RFC4511: the <attrs> field is not mentioned. It mentions, indeed, the <filter> field, but OpenLDAP does not handle this. The behavior you possibly expect is not strictly specified, AFAIK.
I think you have a couple of options:
use ACLs to hide that entry to some specific clients
use a dummy proxy instead of a referral; the dummy proxy could massage
the request/response DNs, and the original server could use ACLs to hide that entry from the results returned to the proxy.
I tought OpenLDAP could support that kind of referrals. Now I think the best option is the best for my scenario.
Thanks a lot
p.