How do I find the windowid of Google Chrome to pass to screencapture -l
You can do that with GrabFS, a screenshot filesystem for OS X. (Download at the bottom of the page. Works fine for me on 10.8.2 with OSXFUSE) It gives you the textures of any processes windows as files which you can simply copy. To update the screenshot, copy again.

Looks like this (converted from TIFF to PNG to save space.): 
You'll likely need to do a little shell scripting to automate this, but that will be easy.