See also the javadoc for BaseDelegatingExecutionFactory. In short you create a new translator that is a subclass of BaseDelegatingExecutionFactory. That is just like any other custom translator development. In your BaseDelegatingExecutionFactory subclass you can override any of the delegation methods to perform whatever logic you want. When you use your new translator (let's call its type delegator) in a vdb, you define the translator by wiring in the delegate:
<translator type="delegator" name="my-translator">
<property value="delegateName" name="name of the delegate instance"/>
<!-- any custom properties you may have on your custom translator -->
Now my-translator has will proxy all calls to whatever delegate instance you have assigned. Note that your delegate instance can be any translator instance and can be configured by it's own translator entry.
Can you let me know where can I find the new documentation?
Check the Developer's Guide in upcoming 7.6 Release.