<p>Essentially the only thing I can deploy to my deployment machine is a JAR file.  I can't install JRuby, nor can I install Glassfish or Tomcat.</p>

<p>Is it possible to package up a Rails application (including Rails, vendored, of course) to a JAR file such that I can do</p>

<pre><code>c:\my_server&gt; java rails_app.jar
</code></pre>

<p>and have it run WEBRick or Mongrel within the JVM?</p>

I'd recommend that you checkout Jetty.  The process for
Embedding Jetty
is surprisingly easy, and it should be possible to give it your servlets from your current jar file.  I haven't used Ruby/Rails, though, so I'm not sure if there are any complications there.
Is it normally possible to embed all of your rails templates/models into a jar inside of a war file for deployment on Tomcat?  If so, then you should be able to get embedded Jetty to pull it from your single jar as well.