How to cache asset files when batch creating thousands of PDFs using wkhtmltopdf?

Question

We are using the excellent wkhtmltopdf to render PDFs from HTML web pages on our website. We need to render and store about 10,000 PDFs in a single batch job. This poses a problem.

Gathering the JS, CSS and image assets required for every PDF via HTTP is going to be very slow - is there a way to cache the files like a browser does across each request ? wkhtmltopdf doesn't seem to have any options to do this when batch creating PDFs.