The runtime installs offline, but it is designed to fetch files from an external host, most commonly runtime.panda3d.org.
However, it should be possible to create a local mirror of the runtime.panda3d.org packages. You could download all the files (with a recursive wget) from runtime.panda3d.org/ to a directory which is passed as a local file:// URL to panda3d via the -u or -M option (experiment to see what works best). Of course, this means you are responsible for downloading updates yourself.