I'm not aware of any way to use the ZOOM menu items from within a program on a TI-81, but a projectile motion program I'm porting requires ZSquare for proper operation.
Basically, the window needs to be set with Δx=Δy with constant x-axis dimensions for projectile launches with angle ≤60° and constant y-axis dimensions with launch angle >60° to keep the entire plot within the screen. The graph corners are already set to the maximum height and distance.

TI-81 graph screens are 64×96, not 63×95.

You could perform the calculations directly and manipulate Xmin, Xmax, Ymin, and Ymax. You'd probably do a calculation like (Ymax-Ymin)/64 to get Δy and figure out how to adjust Xmin and Xmax to get the same Δx, centered on the current location.

I do have to directly manipulate the range variables, but I don't think calculating deltaX and deltaY will be necessary. I'm also limited on the lack of If...Then...End statements. I'm just lost on how to adjust Xmin and Xmax, or Ymin and Ymax.

If I remember correctly, you can store to the range variables as if they were regular variables. They're likely on the VARS screen somewhere.

That's definitely true in the 84 series and the 83 for that matter. Also, since the LCD sizes are so similar, you could most likely just set it to the 83/84/SE Zsquare dimensions and it would appear fine.

The graph corners vary wildly based on the projectile launch; I can't just use fixed coordinate settings. But yes, they work like normal variables.

Although I do have the range variables in the VARS menu, there's no way to use the built-in zoom options.

Press [Zoom] then [5]. You can't use it in a program though.

I've got this pretty much figured out at this point. I set deltaX and deltaY equal to each other, centered at the graph's center.