The @Uri("http://example.jax-rs.org/bar") proposal looks wrong to me because it encourages working against REST by hardcoding URIs instead of discovering them.
Yes, you might argue that entry URIs (aka bookmarks) can be considered to be reasonably stable over time but for my taste, I'd like to see entry URIs to be placed in configurations.
@Uri seems like the client side equivalent to the REST anti pattern of providing service API documentation instead of media type specs.
I favor Client injection.
What about doing this analog to JPA? The configuration of a bunch of clients could be placed into a configuration file and one could obtain an instances my referring to the configuration name:
private Client client;
private Client longTimingOutClient;