Yes. It is possible. It will work as a normal property which you can refer at any place after you get the return value from class mediator. You can get the property 'Message' as you have mentioned but before that you need to assign the result to the property 'Message'.
See the following sample of assign the result,
<class name="in.youtility.esb.custommediators.ReadingAlertMediator">
<property name="target" value="Message"/>
</class>