I just went through Raymarine documentation and tech support for this exact question.
The autopilot core pack (or control head) will only translate and send a very small subset of messages from Seatalk to NMEA. Specific list of messages is in documentation, but really it only includes things related to autopilot operation. So, autopilot cannot act as a general purpose Seatalk to NMEA bridge.
To translate all messages from Seatalk to NMEA (and back) you do need their translatio box. This is somewhat silly, since Seatalk messages are virtually identical to their NMEA counterparts, but that's how Raymarine makes money.