I have an A460 which displays temperature on screen. I would like to have this temperature written to the EXIF data or to a separate log file with a time stamp.
JPEG EXIF is out of our capabilities, but log files and DNG EXIF are possible.
If your post is in the correct place (script writing) then this is very easy. Refer to e.g. the sunset timelapse scripts. Basically you use the get_temperature command and write to a log file of your choice. This should work even in ubasic, with less sophisticated logging of course.
But if you wish this to go to DNG EXIF or to happen in interactive shooting, outside scripts, you need to hack it all into CHDK C code.