In AN14, section 5.3, the third step of sending a reverse message back to the master is:
"3. Send the extended message, appending
the channel ID of the destination master
(configured in step 2) to the data."
I believe this is incorrect.
Instead of sending an extended message to ANT, it works fine to send a normal message on the channel used in steps 1 and 2. After all, the channel configuration is already done for that channel, with the appropriate (device_number, device_type, transmission_type) set.
I ran into this in nrf51422 development, as the nrf51422 SDK doens't include a function to send any "extended message" to the Ant softdevice.
Perhaps the AN14 documentation could be amended to clarify this.