[otrs] external customer backend with filter

Hi all, I've figured out how to set up an external DB Backend for customer users. It shall just have the function of a kind of address book, these customers shall not be able to login. The external backend also is a MySQL-database on the same mashine. What I'd like to do is filtering the amount of addresses in this address book, let's say there exists a field called "access" in the external database table and public addresses do have the entry "group" or "all", how do I configure otrs to only show those entries with "group" or "all" in the field "access". Which "key" do I use and which value should it have? Something like CustomerUserValidFilter for LDAP-Backends perhaps? Thanks in advance for your suggestions. Sincerely