Indeed, it doesn't make very much sense to take very long exposures on a CCD.
You'd usually rather take shorter exposures and stack them (which also help if there is any drift, besides).
That CHDK allows longer exposures than otherwise possible is nice, but don't forget there is
a semi-valid reason why Canon stopped us from taking very long exposures in the first place!
Sensor noise averages out like a charm if you stack several exposures.