Reprogramming the sensor readout would be great. However,
- nobody succeeded in doing it
- the sensor- and readout related parts of the firmware are very hard to follow or understand
- don't expect crop HD video on a camera with CCD (like the A810), the sensor only supports a few readout modes, and this kind of crop mode is certainly not one of them (low light mode is closer, but the theoretically achievable framerate is not high enough for video)
- some CMOS sensors allow reading reading only a part of the sensor
- even if somebody would find out how to do it, large parts of the firmware are not aware of "other" resolutions or refresh rates, and would probably crash