I really don't fully understand how the Hibernate javax.enterprise.inject.spi.Extension implementation is found in all EE environments...

It's the responsibility of an integrator (e.g. WildFly). For non-EE envs (Weld SE, Weld Servlet) we use TCCL if set or the CL of the class org.jboss.weld.environment.deployment.WeldResourceLoader.

...any suggestions on how the javax.enterprise.inject.spi.AfterDeploymentValidation event parameter can be used...

Sorry I have no idea. A static method accessing the "current" Hibernate config?

If a getDeploymentBeanManager method were added to the AfterDeploymentValidation interface,...

This is not possible. AfterDeploymentValidation comes from the CDI API which does not define anything like "deployment" BeanManager or BeanManager hierarchy (it's impl specific).