Unfortunately not yet.

What I thought of doing is to open up the belt button, see that hopefully it isn't cast in a resin block and hunt with oscilloscope until I find some signal that's corresponding to measured heart rate.
then I'd couple that to the mic line of the BT headset and see if I can get it working, depending on the type of signal in the HRM possibly needing some base oscillator there driven bwy it and using it like FM converter... All is speculation yet, It might well be impossible.

Also I am not sure if it's actially 2-way communication, or is the watch part just listening to HRM signal from the button.
If the latter, it might prove to be impossible.

Other approaches; intercept the signal when it is in operation, (with a spectrum analyzer for example...) debug it and build a converter to use with N9, using forex. the microphone input to get the decoded signal to the device.