This is a quick modification to the --server option so that it will accept an optional bind address in the form of host:port. The default value value is still to pass nil (which binds to both the IPv4 and IPv6 wildcards in WEBrick).
http://github.com/benders/jekyll/tree/master at 84142f0, or
Re-submit as a pull request if you're still interested?
Looks like this is active in #686. I'm closing this issue for now.