FWIW, CHDK remoteshoot actually allows you to transfer a sub-image, but only with a vertical crop. The DNG meta-data are not modified (so all the dimensions are the same as a full size raw), the client is expected to either pad the image (as chdkptp does) or adjust the meta data.
This is not available for raw saved to the SD card, though in theory it shouldn't be too difficult to support.
Supporting horizontal crop would be more difficult and often have worse performance. A vertical crop can be done in a single write call. If the rows aren't full width, you need one write per row or some kind of intermediate buffering (or maybe EDMAC tricks).
One of my cameras does an amazing 10 frames/s (for just 1s), but with 1/2 sized frames, it could be a short video clip except in raw.
Raw is not currently supported in the modes that produce half size images (variously called low light, high iso, high speed) srsa did some research: http://chdk.setepontos.com/index.php?topic=10648.0
These modes are (usually?) binned, which with a bayer filter has an unavoidable impact on quality. However, I believe there would be some value in adding support for this in the future, for example the raw meter code could use it for faster timelapses.