We do not want BIP64 to be incompatible with BIP32 in any way. BIP64 is just set of some recommendations for wallet developers how to browse bip32 tree.
Modifying serialization format would break the compatibility.
However we have our solution for storing wallet birth time, which is out of scope of BIP64, but we'll communicate it as soon as we'll write down that idea to some more specific format.
Marek