Retrieve request content with property subs

Is it possible to use an Event Listener to retrieve the outgoing request details AFTER any property substitutions have been applied.

I need to generate an auth header, the algorithm for which requires the actual body of the request as an argument.

Using a beforeSubmit listener, I have managed to retrieve the request content using

submit.getRequest().getRequestContent() however this still has the property names rather than the actual values contained therein and thus results in a mismatch.

Any ideas?