It works!

Token acquisition was failing because of our authentication plugin chaining configuration, with CAS AUTH plugin filtering requests sent using TokenAuthenticationServlet pattern instead of letting custom AUTOMATION BASIC AUTH handle this part of the process too.

To sum it up:

1/ Mapping readable username stored in ldap to a field in user schema called hrUsername following help given here

Method to achieve username translation:

public static String uidFromLogin(String login) throws DirectoryException{
            String username; // output string

            // if given login is made from digits, then return
            Pattern pattern = Pattern.compile("^[0-9]*$");
            Matcher matcher = pattern.matcher(login);

            if(matcher.matches()){
                logger.debug(login + " authenticating");
                return login;
            }

            // else transform
            logger.debug("Getting uid from human readable login");
            DirectoryService directoryService=null;
            Directory  directory = null;
            try {
                directoryService = Framework.getService(DirectoryService.class); // get directory service
            } catch (Exception e) {
                logger.error("Failed to get " + DirectoryService.class.getSimpleName() + " service");
            }
            try {
                directory = directoryService.getDirectory("userDirectory"); // get user directory
            } catch (DirectoryException e) {
                logger.error(e.getMessage());
            }
            Session directorySession = null;
            try {
                directorySession = directory.getSession();
            } catch (DirectoryException e2) {
                e2.printStackTrace();
            }


            Map<String, Serializable> filter = new HashMap<String, Serializable>(); // query filter ...
            filter.put("hrUsername", login); // ...will filter user human readable username


            List<String> directoryResults = null;

            try {
                directoryResults = directorySession.getProjection(filter, "username");
            } catch (DirectoryException e) {
                logger.error(e.getMessage());
            } catch (ClientException e) {
                logger.error(e.getMessage());
            } 


            if(directoryResults.size()==1){
                username = directoryResults.get(0);
            }else{
                throw new DirectoryException("User result has to be unique");
            }

            logger.debug(String.format("Returning %s from %s", username, login));

            return username;
        }

2/ Creating a new java class matching exactly default basic authenticator and using previous method to adapt user username

public UserIdentificationInfo handleRetrieveIdentity(
            HttpServletRequest httpRequest, HttpServletResponse httpResponse) {

        logger.debug(BasicDriveAuthenticator.class.getSimpleName()+" : UserIdentificationInfo");
        logger.debug(httpRequest.getHeader("authorization"));
        String auth = httpRequest.getHeader("authorization");

        if (auth != null && auth.toLowerCase().startsWith("basic")) {
            int idx = auth.indexOf(' ');
            String b64userpassword = auth.substring(idx + 1);
            BASE64Decoder decoder = new BASE64Decoder();
            try {
                byte[] clearUp = decoder.decodeBuffer(b64userpassword);
                String userpassword = new String(clearUp);

                logger.debug("Array has size : " + userpassword.split(":").length);

                String username = userpassword.split(":")[0];
                String password = userpassword.split(":")[1];

                username = Ul2AuthenticationUtils.uidFromLogin(username);// <- here


                return new UserIdentificationInfo(username, password); 

            } catch (IOException e) {

                e.printStackTrace();
            } catch (DirectoryException e) {

                e.printStackTrace();
            }
        }
        return null;
    }

3/ Binding this class to AUTOMATION BASIC AUTH authentication plugin

<component name="fr.univlille2.ecm.custom.drive.authenticator">
    <require>org.nuxeo.ecm.platform.ui.web.auth.defaultConfig</require>
    <require>org.nuxeo.ecm.platform.usermanager.UserManagerImpl</require>
    <require>org.nuxeo.ecm.automation.server.auth.config</require>

    <extension
        target="org.nuxeo.ecm.platform.ui.web.auth.service.PluggableAuthenticationService"
        point="authenticators">
        <authenticationPlugin name="AUTOMATION_BASIC_AUTH"
            enabled="true" class="fr.univlille2.ecm.drive.BasicDriveAuthenticator">

        </authenticationPlugin>
    </extension>

</component>

hope this helps