Is it possible to use a lower ISO instead of ND filters?

It is possible, but there is a limit to the lowest ISO your camera supports based on the sensitivity of the sensor.

Every 1/2 ISO level requires 2x the light. So ISO 15 would roughly be 2.5 stops darker compared to ISO 100.

So while lower ISO would help (in theory), lower-iso + an ND filter would give you a lot more options to work with.

This doesn't add anything new beyond jrista's answer, but to summarize: probably what is happening when you select those ISOs is that the camera overexposes and then divides the numbers by 2 (or whatever). So you lose IQ fairly rapidly.

I would suggest looking up on CHDK website exactly how it achieves those low ISOs. I'm certainly curious, so do report back if you can! :)

The best way would be to shoot with intervalometer to your lowest true ISO (probably 100), expose them right and then later blend them in average mode. CHDK has an average blend for the RAW's or just do them later in PC. The result will be pretty much of a long exposure shot.

BTW: Blending many shots cancels noise because it's arbitrary (random), the more shots blended the more noise gone.

No, with the CHDK, you cannot use a lower ISO instead of an ND filter.

To quote from the CHDK 1.3.0 user manual:

... ISO override gives full control of ISO settings, but does not usually extend the range of available values.

If, however, you are using a camera that has extended ISO settings (i.e., settings that are not accomplished through hardware, but through image processing), you might be able to use the lower settings instead of an ND filter, and the noise will be less, but you will lose dynamic range. And you probably won't be able to go down far enough to do a 10-second exposure in the daytime, as the most common setting will simply give you an additional stop.

I have a 5DMkII with extended ISO settings. The native iso range is 100-6400. I have one "Low" extension (iso 50) and two "High" extensions (iso 12800 and 25600).

The high ISO settings are done by underexposing at iso 6400, and then the image is processed to "push" the exposure higher (like adjusting exposure in post). This will probably add more noise than actually having those settings on the sensor, and in the case of the 5DMkII can cause banding.

The low ISO setting is done by overexposing at iso 100, and then the image is processed to "pull" the exposure lower. Something you can also do yourself in post. This can reduce the noise in the dark areas, but will reduce the overall dynamic range of the image and possibly lose detail in the highlights.

On some Canon dSLRs, partial stops are also done by digital push/pull, which is why you sometimes hear the advice to use ISO multiples of 160 if you want to reduce noise (i.e., the -1/3EVs are pulled, the +1/3EVs are pushed), but this is dependent on your camera model. IIRC, Nikon dSLRs actually use gain across the sensor so there's no "stairstep".

There's no such thing as a free lunch. You have to pick your tradeoffs. The iso 50 setting on my 5DMkII is something I'd only use if that extra stop on the shutter speed makes losing the dynamic range worth it. Typically, it doesn't.

Have you considered taking multiple photographs and averaging them? This works perfectly as long as you don't have any clipped pixels or the pixels are clipped in the same location for all the photos. This method does come with a disadvantage that there is a gap between when each photo is taken so for moving objects the result will not be the same but this method is great for reducing noise for still objects.

The reason this doesn't work as well with clipped pixels is because the real value is unknown.