<Set name="fileName">path/to/myaccess.xml</Set>

part should be *inside* the definition of the request log implementation, should look like this:

<Ref id="requestLog">
	<Set name="requestLog">
		<New id="requestLogImpl" class="ch.qos.logback.access.jetty.RequestLogImpl">
			<Set name="fileName">path/to/myaccess.xml</Set>
		</New>
	</Set>
</Ref>

With the current docs Jetty doesn't even start up.