If you're looking for a full-screen zoom for the Springfield and Kar98 Sniper like in the video above, I agree that it's probably impossible.
There's a shader somewhere in MOHAA's code that forces thick black bars on both sides of the screen in order to make a perfect square in the middle.
That square is divided by 4 tiles, where a zoomoverlay.tga file for only the top-left tile is copied and rotated 3 more times to fill the middle square and make a circular sniper scope when zooming.
Here's a .pk3 that makes that textures/hud/zoomoverlay.tga file blank (from Pak1.pk3), but once you zoom in with this mod, you'll see that the black bars on the sides are still there (can't be removed in AA).