I am currently using Instruments via a bash script to initiate the command-line interface to start up runs of the Automation plug-in.
With 4.2, this worked well en
Here's a wonderful command that may work for you:
security unlock-keychain -p [password] "${HOME}/Library/Keychains/login.keychain"
It's the command-line way to gain access to a keychain on the Mac. I haven't tested it with Automation, but it's how I've integrated my iOS builds with Jenkins.
(Replace [password] with the password)