Reprogramming a computer mouse's internal IC?
Specifications for this particular chip (SPCP168A) don't show any explicit serial/SPI port. Also, there is no indication in blogosphere that this particular chip has any field firmware update options. So it is very likely that it runs from a mask ROM.

However, there are other optical mice on the market, they are based on the original patented Avago technology. The main chip ADNS-2051 does have an interface to a MCU, both some serial, and "quadrature outputs". There are some hack examples that use this chip as 16x16 video matrix using Arduino.