ResgisterBusVolumeCallback in Unity

Question

With the 2015.1 addition of the Mute Bus For Background Music feature, we find ourselves needing to know the volume of our Music bus (for things like accurately setting the Settings UI to reflect when the Music bus is being muted for User's BGM). The best thing I can find online is the ResgisterBusVolumeCallback, but I can't seem to find the call anywhere in our Unity integration. It does show up in the Mac binary, but no way I can see to call into it from our game code.

https://www.audiokinetic.com/library/2015.1_5418/?source=SDK&id=namespace_a_k_1_1_sound_engine_a48a069a39cd20d04fbbd8782df3ee386.html#a48a069a39cd20d04fbbd8782df3ee386

How can I register for these callbacks, or is there a better way to get a bus's volume in the Unity Integration?