Anything over 180° (and up to that, with massive increasing inefficiency) means you can no longer render a rectilinear viewport. You need to start rendering multiple actual views per eye with their own viewports (not just shifted frustums like with Lens Matched Shading). This is the sort of thing where you start having to customise the game engine to get it to do what you need rather than shifting around the existing view.
Also, that documentation is just for the application-side API to talk to SteamVR. For the drivers to talk to SteamVR there is a
different API.
However, there does not appear to be a way to pass multiple projection matrices per eye available, so OpenvR would have to be 'hacked' to add the functions needed (like with the Kinect body tracking demo where 3 points on the skeleton were turned into virtual Vive trackers because OpenVR has no way to pass a skeleton post). One way would possible to present the HMD as two HMS that happen to be in the same physical location but with different optical geometries. A huge pain to keep things synced up though.
In order for things to be done 'properly', like any extension to OpenVR it requires you to wait for Valve to implement what you want then update SteamVR to support that extension.
like I needed an excuse.