I mean no disrespect with this reply, however...
1 ) The ability to overclock the CPU is already part of the GP32 SDK. Many programs who would benefit from overclocking already support it.
2 ) The maximum for the CPU is 200 MHz. The maximum for the RAM bus is 188 MHz. The maximum for most people's RAM is 166MHz. Therefore, the maximum for most people's GP32s is 166MHz.
3 ) The higher the CPU is clocked, the faster the batteries are eaten. If the CPU was able to be overclocked to 300MHz with the current design, your GP32 would probably last all of one or two hours.
4 ) An "overclocking program" is not possible with the default firmware (GP OS). Every program sets the clock speed to a certain level upon start. Even if not set beforehand, it will default to 40MHz.