As workaround I succesfully did the following:

- Install somewhere (preferibly on your local pc for a better security) an apache http web server (e.g. installing it from Xampp)

- Configure the following Proxy rule replacing the encoded string with the user-id+password combination used for the basic authentication, and the http://myserver... with the URL of the actual installation
of the protected openrdf-sesame.war
(see http://en.wikipedia.org/wiki/Basic_access_authentication for a description of the encoding mechanism).
e.g.:
<Location /openrdf-sesame>
    RewriteEngine on
    RequestHeader append Authorization "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ=="
    ProxyPass http://myserver:8080/openrdf-sesame
    ProxyPassReverse http://myserver:8080/openrdf-sesame
</Location>

- unpack openrdf-workbench and change the init-param "default-server" pointing to the proxied url above described (in the example we assume a default apache installation listening on the port 80)
  <init-param>
<param-name>default-server</param-name>
<param-value>http://localhost/openrdf-sesame&lt;/param-value>
</init-param>

- deploy the modified version of openrdf-workbench on a local app-server instance