Cool idea, but I don’t think it’ll work because serial mice send data at 1200 baud, which is too slow for a PIC18F8520 running at 40 MHz. Here’s a link to the protocol specification: http://www.itacsystems.com/rs232mousespec.htm
I know its really hard to find any purely serial mice these days. They need them for lighting boards and my brother always had to special order them, but only the ball style, cause only a few companies still make them. Good luck finding them.