Hi Paul,
I don't think there is an external mailing list for chromium OS EC development.
To see the manifest which the repo utility requires you need to setup the Chrome OS chroot environment first (aka cros SDK), I am not sure if you did that, but this is a pretty expensive procedure, requiring a lot of disk space and time.
You should be able to clone the EC git repository and run pre-upload checks and push patches to the EC gerrit server, but you'd need an account on the server for that.
To be honest with you, I don't know the exact procedure to follow to get the account created, you could inquire at
chromium-os-dev@chromium.org
the entry threshold is a big high, but I am sure the Chome OS EC team would appreciate your contributions.
--vb