User fingerprint sensor to control media playback
You guessed correctly. Speaking as a developer, there are no APIs Apple provides to interface with the fingerprint reader directly. Even the process of asking to store credentials goes through a system API that really only gives you a success or failure result and an optional reason. As their docs put it:


  The context handles user interaction, and also interfaces to the Secure Enclave, the underlying hardware element that manages biometric data. 


You never get to see or interact with that data.

Also, the fingerprint reader on Touch ID-enabled Macs functions as a power button, so there's no way to remap that key to do anything else useful either.