I would like to use a base directory relative to ${WORKSPACE}.
In other plugins, I can just write ${WORKSPACE}/foo/bar/baz.
This does not work on the wl-deployer.

It would be nice to have this in all configuration options (e.g. global + per-job options)

See first answer here for a hint on how to implement this:
http://stackoverflow.com/questions/23800416/jenkins-inject-environment-variable-in-jelly-field