This turns out to be badly documented and the method appears to have changed repeatedly, but once you find the right resources it works well.
Downloading the standard Chrome offline installer and trying the usual command line help options wasn't much use. Instead of reporting a usage page, it responds to unrecognised command line options with
"Google Update installation failed with error 0x80040c01". Helpful!
There's lots of info on the 'net, but it appears that the Chrome installer has gone through several iterations, so much of the advice is wrong.
/installtriggers a non-interactive install with a GUI ... except that it still launches the interactive installer for Adobe Flash Player if it isn't already installed. Not helpful.
It's actually easy to deploy Chrome centrally, but you wouldn't think so from searching the Google Chrome help pages and site.
The key thing you need is the Chrome FAQ for Administrators page on Chrome for Business. This finally explains that there's an MSI installer. Download the MSI installer.
Now you have a nice sensible MSI named
GoogleChromeStandaloneEnterprise.msi. You can invoke installation with
msiexec /i "GoogleChromeStandaloneEnterprise.msi" /qn /quiet /norestartlike any other MSI. As an MSI, it can also be deployed over Active Directory using Group Policy.
It's a bizarre that Google make this so hard to find.
By the way, if you're doing WDS deployments, you owe it to yourself to use MDT thin installs. MDT will merge drivers into PE images for you, create installation scripts, and largely "just works". It can do things like auto-update your newly installed images from a local WSUS server, doing all that tedious rebooting and updating for you completely unattended. Since Microsoft has no plans to release a Service Pack 2 for Windows 7, that's going to become rather important as the update load continues to grow. Until Microsoft backflip and release SP2 anyway, of course; we've heard all this before with XP and SP3. Use MDT 2012.