Hi Andrew,
Your transaction launcher entry is connect to an handler class (you can see that in customizing). I'm not totally sure about this but I truly believe one of the class methods is called when you return from ECC transaction and you'll be able to intercept that data there.
Can you try to search in there?
Kind regards,
Garcia
Correction: Sorry, after I've replied checked that the interface method you mentioned is the one use by this transaction handler class, so you're already there. Ignore my answer or try to investigate method GET_AND_ADD_ENTITY_TO_AC of your transaction launcher class, I need coffee 