You can do that on interceptors, not on transformer.
However, that seems an easy fix...
The only places Transofmer is used is on DivertImpl and BridgeImpl...
You could maybe change the code to do this on DiverIMpl:
if (transformer != null)
copy = transformer.transform(copy);
if (copy != null)
postOffice.route(copy, context.getTransaction(), false);
On The Bridge.. we probably need some tweak as we need to make sure the message would be acked.
Maybe you could add a JIRA?