The tool would execute during travis builds (once per entire build) and use special fixtures that provide meaningful data for screenshots. Plugins would provide the special fixtures and instructions on what elements to take screenshots of, what to do before taking the screenshots, etc.
The tool could use the existing screenshot testing code, but that code is more for testing than it is for browser automation. It may be better to use casperjs.
Whatever code a plugin needs to provide to make use of this tool should be minimal.
There are a few uses to generating these screenshots automatically: