I've added dev-java/stax2-api to java-overlay for use by davmail. Please see the URL. These classes used to be bundled together with wstx (woodstox) but upstream indicates some degree of separation and davmail does not require the rest of wstx. I have committed a newer version of wstx to java-experimental that depends on this package. I chose java-experimental because some tests are failing and I don't need wstx right now.
Version 4.0.0 now in the tree.