Yes, sort of. If you put the ISO itself on a HTTP server somewhere, you can use it that way. Our upgrade download is a zip file so that we can include upgrade instructions, but you can unpack it to a server that you can get a URL to on the internal network (heck, even external).
Would it be helpful to have the ISO somewhere you could refer with a URL? We kind of assumed people would want to pull down the zip and do it themselves, but I can look into whether we can host a second download that's just the ISO.
zip should be fine. I thought I tried that but must be missing something. I'll try again with next release.