The following is a list about future development possibilities with a hijacked FileWriteTask
All file access should be done before the "open" method of the task. After that, a system-wide lock on any other file access takes place. Disabling that lock would probably be hard as at least one other task (FileScheduleTask) is involved.
Get a resized YUV version of the last picture taken
One has to set the desired image size before the shoot. As at the time this task is called the LCD / video out is not yet enabled (needs proof), the undamaged uncompressed source of the jpeg image should be available. The address needs to be found, the task doesn't know about it.
Get the JPEG image
The filename, the jpeg buffer's address, its size is available. It could be used to transfer this image over PTP. Need an idea how to do this. An option could be to disallow saving the image to the card.
Modify the EXIF data
The EXIF buffer is known (it's temporary, but valid when the image is to be saved). This would be a great opportunity to append the CHDK metadata normally only present in the DNGs. The original Canon values should probably stay untouched, some comment field could be used to write the additional data.
Rename the image before saving
May not make much sense. Could result in the camera not recognizing the (renamed) pictures.
Here's a possible use: http://chdk.setepontos.com/index.php?topic=5685.msg55321#msg55321