I've just discovered how to enable Bash autocompletion for the wsk command line tool!
$ cd /usr/local/bin $ wsk sdk install bashauto
This will create a file called wsk_cli_bash_completion.sh in your /usr/local/bin directory.
Now, source this file within your .bash_profile or equivalent:
$ echo -e "\n# OpenWhisk autocompletion\nsource ~/bin/wsk_cli_bash_completion.sh" >> ~/.bash_profile
Start a new terminal window, (or source ~/.bash_profile in your current one) and you can now press the tab key after typing wsk to see the available options.