Is there any way to set a custom quick look image in OS X? I'd like a command line tool or applescript to change or embed a custom preview for a zip file using a jpeg.
That's not how QuickLook works. Quicklook has processes that run in the background to interrogate a file for its filetype, then look for a generator for that file type and the generator returns either HTML-based or image-based content.
There are programs that will show a listing of files inside a Zip (or other archive files) but you can't "embed" a preview image for Quick Look, at least AFAIK.