It's actually pretty close. It's theoretically possible for mrjob to run a script written in any language as long as it supports a simple stdin/stdout protocol. We just don't have any users dedicated enough to implement that stuff.

Also, you can write a very small Python script that uses subprocess to call your script. That way you can still use mrjob to set up all your dependencies and handle AWS for you. If anyone is actually interested in that sort of thing, I'd be willing to write a tutorial.