This is totally a WS issue, I'd ask on the JBossWS forum.
Got it semi-working, now it's a JBoss Tools issue(?). POJO WebServices are apparently mapped as Servlets and are pre-processed when deployed.
The problem is that when I have my web service in the action src dir I get a
[JAXWSDeployerHookPreJSE] Cannot load servlet class: org.domain.WS.session.Echo
because the scanning is done by another classloader that doesn't know anything about the WEB-INF/dev folder.
So it looks like the hot-deployment in JBoss Tools Seam Web Projects is incompatible with POJO WebServices (unless someone has a workaround).
To be expected. Only Seam knows anything about WEB-INF/dev.