I've just loaded CHDK v1.1.0-2247 (from main download site) for my A800 v1.00c, DNGs have been decoded normally - no purple mask. Although both DNG and exported JPEG has weird black/purple dots, around 3px in diameter. On DNG dots are blurred, visible when highly zoomed. Those on JPEG are more sharp, obviously. JPEGs created by Canon firmware are free of those dots.
Good thought is such, those could be just noise.
I haven't succeeded in reducing that noise while decoding RAW in any way. I even blurred the image quite a bit and those dots seemed to be purple...
I created badpixel file beforehand, count is over 12300. I even removed badpixel.bin from SD card, no difference.
I use digiKam to manipulate images extracted from my camera.
I'm impressed and happy to use CHDK
.
Edit:
I took another picture with more light and that dots are almost invisible. Again Canon JPG is free of that noise.
Edit 2:
Now I have hard time reproducing that effect. It's now less visible. I've recreated badpixel.bin in better light conditions, so it's better now.