Would these IBLT tx relay techniques also help defeat traffic analysis? Seems to me that they might, which would improve privacy against attackers like the NSA.
Yes. They would frustrate traffic analysis at least if 'constant size getdata' were also implemented-- assuming other mempool leaks were closed. (like the mempool p2p message), and if the traffic were encrypted (e.g. over tor or other encrypted transport) or if content were not visible to the attacker (e.g. for pretextual compliance with the law).