I found out more about this.
JBoss Tools should use Seam's JAR scanner to scan for Seam components, just like Seam itself does. But it doesn't. It looks at the
file to get a list of Seam components. That list is automatically managed when Seam entities are added to the project.
So, to get auto-complete etc working with external JARs, you need to add the components
by handto the metadata file.
If I'm understanding correctly what is going on, I don't think this is the right way to do it. Why not use Seam's own jar scanner and components.xml parser to detect components?
Wrong forum, please ask on the JBoss Tools forum.