found a solution here: http://javahowto.blogspot.de/2006/06/4-ways-to-get-ejbcontext-in-ejb-3.html
do a JNDI lookup for EJBContext in your interceptors doIt() function:
InitialContext ic = new InitialContext();
SessionContext sc =(SessionContext) ic.lookup("java:comp/EJBContext");
this seems a bit hacky to me, but none of the other possibilites worked for me. In any case I got some kind of "no bean found in context" error.
as this post is a little aged, I suppose you found a solution already, but maybe this is helpful for others.