base64binary is hardcoded so it can’t be changed easily
I can suggest to use OnCustomResponseEvent. this link contains some example that will be useful for you.
Note: this event only works for unhandled (i.e. unknown) requests.
you can process here link like http://localhost:8099/mywsdl
this event should catch this request, change path and pass modified data to MainProcessMessage.
later you can grab aRequestStream and modify data as you like.
the same can be done with usual method calls (http://localhost:8099/mysoap) - you can modify aRequestStream and change hexBinary data with base64binary