Not possible with Alyx. Also, the fall speed in Alyx is really slow. Alyx is made with VR beginners in mind so it has a lot of features designed for newbies. This is also why teleport movement is default. Portal 2 is not made for vr which is why it can have fast movement and flinging. If you want that, and better looking portals, i suggest portal 2 vrmod though there are also a lot of things that make this mod better. The screen going black isnt really to decrease motion sickness but moreso to make it look like you are actually being flung rather than what is happening behind the scenes

We are looking into possible solutions through scripting, but it's very complicated and we'll need time.

The Holy Grail!! :) I Hope someone gets physics inertia through a seamless endless path :)