Painting on filter masks works, you can test that by replacing the code in KisFilterMask::apply with the same code from the transparency mask. After doing that the filter mask works as a transparency mask (suprisingly without the crash from the transparency mask), so I supect that something goes wrong when the filter is applied.
Well, i'm trying to QImage::save() selection for both types of masks and they differ. FilterMask's is fully white, Transp.Mask's is black.
I'm investigating the reason of that now.
From the other hand, if i manage to work both masks, it won't be enough for a usual life as painting on masks DOES NOT support indirect painting (as far as i understand).