Add a test to answers this question:
Does an extension have to be in an bean archive in order to be loaded?
Section 11.5 states:
"An extension is a service provider of the service javax.enterprise.inject.spi.Extension declared in META-INF/services."
If one assumes that "service provider" refers to the term defined in the jar specification , then one would conclude that an extension does not have to be in a bean archive to be recognized (these are orthogonal concerns).
A test for this scenario is available in the OpenTCK project and may be ported to the CDI TCK.