Rocky,
Technically, everything can be installed offline, provided you have the necessary packages. Of course, will need to validate that statement. (As you know, there are numerous attributes that need to be set in order for the generated HTML document to work offline, but that's a separate topic.)
Asciidoctor (core)::
If you're on a Linux box, you can download the Linux package (rpm, deb, apk) and install that offline instead.
fopub::
Once you run fopub once, it should operate offline after that. The initial run uses Gradle to collect all the dependencies and setup the installation structure.
(If we put in a little bit of work, we could create a distribution for fopub that has the files already downloaded and in place).
Asciidoctor Diagram::
The chart extension really isn't packaged at this point, so I don't really have instructions for that one. Eventually, we'll want something like an asciidoctor-chart gem.
-Dan