Hello David,
There is not currently a way to use LibreJS as a command line tool to test a URL's licensing of its JS.
As you asked, the only way to currently use LibreJS is as a browser extension.
We are currently reworking LibreJS to make use of the WebExtensions APIs which will replace the the legacy XUL APIs in Mozilla Firefox, and then by extension GNU Icecat.
I'm just about to publish something about something around this to the news feed on Savannah for LibreJS [0], so you'll be able to read more there in a few minutes.
-zach