Stereo mode is already available. A button automatically appears on compatible devices such as smartphones or tablets. If you don’t see it on the device you use, can you please tell me which device you use, and which browser?
Oh, okay, I totally misunderstood the question, sorry!
For the moment, only equirectangular panoramas can be interpreted by WP Photo Sphere. Other projections, like the one you describe, are not. Maybe they will be in the future but I have other big things to implement before.