Jason or Anil, is it possible to give an example on how this module="" attribute is actually supposed to be used. I have no idea.
I tried the following in the standalone.xml:
<security-domain name="MyCustomSecurityDomain" cache-type="default">
<authentication>
<login-module code="com.my.CustomSecurityLoginModule" flag="required" module="SecurityModule.jar.com.my.CustomSecurityLoginModule">
<module-option name="password-stacking" value="useFirstPass"/>
</login-module>
</authentication>
</security-domain>
(Adding the module to jboss_install_dir/modules/org/jboss/as/remoting/main and modifying the module.xml file at the same path works indeed as Daniel Jipa pointed out (Thanks). It's however not the most elegant solution of course but at least a workaround to continue coding.)