I have an old ass ipad here running ios 9.3.5v Its about the same size as a mousepad. What if you could make an app that would make a texture on the screen that could be readeable by optical and laser mice? Maybe different sensors pick things up differently so you would need to scramble the texture in different ways to get it to be read better. Yes, you could just do images, but instant generation of a texture that is randomized would be handy. Could just be bumped in the cydia store or something.
I think you’re overlooking all the optical properties of the glass, the digitizer and the polarized filter that are between the mouse sensor and the screen.
Also, if it’s an old iPad, most likely it’s not using screen, digitizer and display fused so the air gap between the glass and the screen it’s pretty big which will surely hurt precise tracking.
In my opinion it won’t work that well, but you can surely give it a try. I don’t think it’s very hard procedurally generate a texture and making it into an app.
It’s an iFixit picture and it shows the glass/digitizer separated from the screen so the gap it’s going to be pretty noticable compared to an all in one unit, like the one you most likely have on your smartphone right now.