Looking at this video and heartcast it does not require special heartcast Watch app, they just say start activity. You might not be directly reading HR from watch but 1-second delay is fine. If you wanted to use heartcast it would require two devices but if we read Apple watch’s HR “signal” just like heartcast, that looks like it will work just fine. If we wanted to attempt to do HRV analysis or something, I assume it would require a direct connection.