The ruby options are a good way to do this. Specify that the $LOAD_PATH have /usr/bin.
-Idirectory specify $LOAD_PATH directory (may be used more than once)
It is what it is there for, after all.
Another thing you can do is push the directory onto the $: global psuedo-variable.
$:.push("/usr/bin")
This would fit wherever you have your environment customization in your application.
I would probably do this for the Rails Application itself, as that is where you are concerned about it. It is what you have control over.