I don't really know much about CHDK, outside of that it lets you program P&S cameras. When it comes to ISO, though, the story is rather tricky. It may be possible to program ISO down to 15 with CHDK, but it may not really do you any good. There are "real" ISO settings, and there are "artificial" ISO settings. A real ISO setting would be something like ISO 100, which is usually the base ISO for many cameras (some have a base ISO of 200). This is an analog setting, adjusting the ISO to any of the "real" native settings will usually work by changing the analog readout of the sensor.
In contrast, an artificial ISO setting is either achieved by making other "behind the scenes" changes to other settings on the camera, or by "digitally enhancing" the nearest real ISO setting. In the case of "behind the scenes" ISO, rather than actually changing the ISO when choosing, say, ISO 50, the camera may actually reduce the exposure time at ISO 100 instead. In the case of "digitally enhancing" ISO, the camera may use ISO 100 when you choose ISO 50, then apply a digital filter to the resulting image to make it appear as though it was shot at ISO 50.
Generally speaking, using artificial ISO settings is undesirable. You generally don't really know what is going on behind the scenes, and if the camera is changing settings to make it appear as though you are really using ISO 50, it may adversely affect your image, preventing you from capturing the scene you really wanted to capture. Most of the time, it is best to use "real" ISO settings that change the analog readout from the sensor. If you need to lengthen your exposure times beyond what the native base ISO setting allows (usually ISO 100, sometimes ISO 200), then filtration is usually the best option.