Multiple Schema and extensions

Is it the case that extensions can be added to only one schema?  If so, what is the recommended practice for accessing
afunction from an extension in multiple schemas? 

Is it *ok* to load the extension in the pg_catalog schema so functions can be accessed by unqualified names?  Is it
*better*to have a separate schema for functions and use qualified names to access?  What is the general accepted
practice?

As a side note:
ALTER EXTENSION foo ADD SCHEMA bar
What the heck does this do?  AFICS, it has no effect on the visibility of an extension function in the *added* schema.

thanks
alan