I don't have a complete answer, but....
The behavior is defined in Jetty. It looks like ServletHolder.setInitOrder(0) is the programmatic way of forcing a servlet to initialize during startup.
The XML you see below....
<New id="context" class="org.eclipse.jetty.servlet.ServletContextHandler">
<Set name="contextPath">/</Set>
<Call name="addServlet">
<Arg>com.peoplepattern.streams.execution.api.RootResources</Arg>
<Arg>/*</Arg>
</Call>
<Set name="sessionHandler">
<New class="org.eclipse.jetty.server.session.SessionHandler" />
</Set>
</New>
...is equivalent to calling...
servletContextHandler.addServlet("com.peoplepattern.streams.execution.api.RootResources", "/*");
There is a 2nd method on ServletContextHandler that allows you to pass in a ServletHolder...
addServlet(ServletHolder servlet, java.lang.String pathSpec)
So just taking a stab at it, maybe something like this would work? (I'm no expert on the jetty.xml files, so I have no idea if this is valid syntax)....