I confirm that the actual path is invisible now to the clients, using what g suggested:
On the nfs server I have

df
/dev/drbd0            20635024   4964288  14622532  26% /drbd0
/drbd0/nfs       20635024   4964288  14622532  26% /nfsapp

and
/etc/exports
/nfsapp 10.1.1.0/255.255.255.0(ro,all_squash,mp=/drbd0,async)

while fstab of client is
nfsserv1:/nfsapp    /nfsapp            nfs    defaults,noauto        0 0

and a df command on it now gives:

nfsserv1:/nfsapp      20635040   4964288  14622560  26% /nfsapp

So the original path is not visible at all to the client.

Thanks again g (whoever you are... ;-)

Gianluca