Originally Posted by
adamrice
Aside from the problem of the market size probably not justifying the R&D, the Apple Watch just doesn't work that way. It basically acts as an iPhone peripheral; I am pretty sure it doesn't send standard Bluetooth signals that a head unit could use—it does connect to headsets by Bluetooth, but that's a different device profile. Everything else is mediated through the iPhone, which is why you can't use it as a HRM, either, but there is are iPhone apps that will retransmit an Apple Watch's heart-rate data over Bluetooth.
This doesn't seem to be entirely true for the later models. You can make/get calls on the watch without a phone.
Even if it doesn't allow it currently, there's no reason the watch couldn't send HR via BT (it's just a profile). If it can do all the phone stuff, it would be able to transmit HR.