it's not really possible to do that.
you could probably add an portlet instance interceptor that would take the portlet instance name and set it as a request attribute, really quite easy to do but would be targetted for 2.6 for instance as you would use an internal API of the portal.
You can look at the org.jboss.portal.core.model.instance.InstanceSecurityInterceptor to base your work on.
This interceptor is then declared in jboss-portal.sar/META-INF/jboss-service.xml
BTW, the instance name can only be known at runtime and not in init() method as a same portlet could be invoked by 2 different instances.
Thanks for your help. I've created the portlet interceptor and added it to the stack in jboss-service.xml, as you suggested; and made the portlet instance's name a request attribute. And it worked!
Thanks once again.
An alternative to get portlet instance data is to put them into the portlet-instances.xml preferences.