After some discussion with
Tihomir I have added an @XmlConfigured annotation to seam-xml. This prevents the bean from being installed, so it is possible to create beans with ambiguous dependencies and allow the user to pick which one they want with XML configuration.
The problem with this is that if you want to use it you now need a compile time dependency on seam-xml just to get one annotation. Should we have a @DoNotInstall (or whatever) annotation in weld-extensions instead, or is a compile time dependency on seam-xml ok?
Tihomir has pointed out that @XmlConfigured makes it much clearer what is going on, and tells anyone reading the code to look in beans.xml, and I am inclined to agree with him, but I would like to know what everyone else thinks.