Here i have added sample synapse configuration to get carbon server home property and return it as response. Add this configuration and invoke proxy service. You will get carbon.home as response.
<proxyname="EchoProxyTest"
transports="https http"
startOnLoad="true"
trace="disable">
<description/>
<target>
<inSequence>
<sequence key="responseTest1"/>
</inSequence>
<outSequence>
<send/>
</outSequence>
</target>
</proxy>
<sequencename="responseTest1">
<script language="js">var carbonHome = java.lang.System.getProperty("carbon.home");
mc.setPayloadXML(<serverHome>{carbonHome}</serverHome>);</script>
<headername="To"action="remove"/>
<propertyname="NO_ENTITY_BODY"scope="axis2"action="remove"/>
<propertyname="RESPONSE"value="true"/>
<send/>
<log level="full"/>
</sequence>