I think back in the QBasic Period, I did something similar. I used PCOPY, back then, to move my cursor, copy it , and return it back to the original screen. I think it was a little glitchy, due to the speed a mouse can be moved. It was fine with slower movements. I hope QB64 has a better solution to making a custom mouse. Since I stay in SCREEN 0, the only thing I normally need is the LOCATE statement to change the appearance of the cursor from an underline to some type of rectangle or square.
Best wishes with the custom cursor idea!
Pete