Won't work for anything that uses relative mouse movement or keeps the mouse position internal.

That's true... A mousse doesn't send coordinates, but individual "ticks" indicating up, down, left or right move.

So I think the solution is to search where in memory are the byte(s) that indicates mouse cursor location in the game, and make a trainer for that, so pressing a custom key will change those "mouse cursor location" bytes and send the click... Do you know if this can be technically be done?

on game for game basis. probably.

ripsaw8080 make me this loader (file attached) for Elvira 2: The jaws of Cerberus. It includes the source code (asm) if you want to see it.

The cursor keys are mapped to the forward, back, left, and right buttons, as you'd expect; pgup and pgdn are mapped to the up and down buttons, end is mapped to the "turn around" button. You can also use the equivalent keys on the keypad with numlock off, and the 5 key in the center is mapped to the "turn around" button because it seemed fitting.

To use the program just put it in the Elvira 2 folder and run the game with cerberus.bat as usual.

It perfectly works, now playing Elvira 2 is faster and more handy. Again, thanks ripsaw8080.