Hum, the translucent overlay allows to a user who's not on my local db to login -> I don't want to write up ACLs for some thousands of users!!!
Well, I guess I did find the way that best suits my needs using the "rwm-rewriteMap" command :

########################

rwm-rewriteMap   ldap   uid2dn   "ldap://remote-ldap.example.com:389 /ou=people,dc=example,dc=com?dn?sub"

rwm-rewriteRule   "^uid=([^,]+),cn=users,dc=local,dc=example,dc=com$"
                           "${uid2dn((&(objectClass=posixAccount)(uid=$1)))}" ":@I"

########################

That does all I wanted to ^^

Thanks for the support,
Frava.