raw is actually relatively simple. Most cameras will have to have the pre-debayer/pre-jpeg readout of the sensor somewhere. As long it isn't completely hidden behind some opaque hardware, find it, write it to a file, there's your raw! This not to make light of work Vitaly and Grand did early in CHDK, getting to the point where you can do that is a big job.

A bit OT, but CHDK raw doesn't necessarily buy you a lot. You get a little more dynamic range, and you can do white balance after the fact, but it's only 10bpp (some newer cameras are 12 bit, but only about 10 bits are significant. wth Canon ?), and the sensors and optics are major factors in quality. It's useful in some specific situations, but raw isn't going give your P&S snaps the same quality you get from a much more expensive camera that has native raw capability.

The stuff done by the Digic DSP (rather than the plain jane ARM946ES that runs the OS, UI and CHDK) hasn't been deeply explored. You might be able to get color accent back, if your hack-fu is sufficient. The fact it hasn't been done isn't by itself a sign that it is impossible, it more frequently means no one has spent a lot of time in that particular area. Of course, if you did spend the time, you might find that it is in fact not possible. The canon firmware code is complicated enough (~3MB of machine code and data for the a540) that there are huge areas that no one has really dug into. The features that exist in CHDK are the result of someone scratching a particular itch.