I would like to rename my raw files using timestamp but a second granularity is insufficient.

I noticed that libraw_imgother_t.timestamp is a time_t, which is limited to seconds. The timestamp value comes from cases 36867 and 36868 in the tag switch (parse_exif in dcraw_common.cpp). These values correspond with the following Exif tags:
0x9003 36867 Photo Exif.Photo.DateTimeOriginal Ascii The date and time when the original image data was generated. For a digital still camera the date and time the picture was taken are recorded.
0x9004 36868 Photo Exif.Photo.DateTimeDigitized Ascii The date and time when the image was stored as digital data.

The Canon CR2 files contain the corresponding Exif tags for sub-second values:
0x9291 37521 Photo Exif.Photo.SubSecTimeOriginal Ascii A tag used to record fractions of seconds for the tag.
0x9292 37522 Photo Exif.Photo.SubSecTimeDigitized Ascii A tag used to record fractions of seconds for the tag.

Are there any plans to add a sub-second value to the libraw_imgother_t structure in a future version of LibRaw?