This may depend on the XMPP server you’'re using. However, I think that most XMPP servers will ignore resource when delivering offline messages. Your best bet may be to use the more fully-featured offline message JEP: http://www.jabber.org/jeps/jep-0013.html
That should let you only retrieve offline messages that you’‘d like to get. I’'m not exactly sure which servers support that JEP. Smack would need to be extended to support the protocol, however.
Check out JEP 13 in the link in my previous message. The old offline message protocol would just flood you with new messages as soon as you sign on. The new JEP (13) defines a framework that lets the client determine exactly which messages to deliver – this is what you could use only get messages intended for a specific resource.