If you do not use rvo obstacles,you should be able to just edit the rvo controller script to trick the core rvo agent to think that it is moving in the xz plane when it is actually moving in the xy plane. However if you need rvo obstacles, some more edits need to be made.
You would need to take a look at the RVOObstacle.cs and translate from XY coordinates to XZ coordinates inside the AddObstacleInternal method. At least I think that should work. Though possibly a better way to do it is to modify the GetMatrix method to convert from XY coordinates to XZ coordinates.