I know this is an old post but I've also had the same question as the poster and I have found the answer. So, I'd like to share. I hope it helps other folks with the same request.
For folks wondering why someone would want to reduce the FOV, let me explain. The FOV tangent multiplier in Quest2 reduces the FOV. This means the area outside the reduced FOV is not calculated. This saves tons of GPU time even with small FOV reductions. For example, using 75% FOV tangent multiplier calculates only 56% of the full resolution. This means you can avoid using lower render resolutions, hence maintain the sharp image while reducing the calculation load. The major trade-off is that through the headset you see a rectangle outside of which is black. I was fine with this trade-off to get sharp images.
On the HP Reverb G2, similar adjustments can be done with the help of OpenXR Toolkit. OpenXR Toolkit goes even further with a function called Fixed Foveated Rendering (explained here: https://mbucchia.github.io/OpenXR-Toolkit/fr.html). This feature renders outside areas in lower resolution while keeping the sweet spot at high resolution. In my opinion, this is better than the FOV tangent multiplier in the Quest2 because it avoids the rectangle in black. Additionally, OpenXR Toolkit allows you do reduce FOV in a separate setting if you so choose.
The below video explains how to set up OpenXR Toolkit. It also explains how to play SteamVR games on OpenXR, which is also helpful for other VR games. Thanks to Ham VR for making this video on YouTube. It made a huge improvement in my VR experience with the G2.
