Yes, this is possible and infact has been done already,
the script to do this comes bundled with Blender: see blender-thumbnailer.py.

This script runs without Blender, eg extracting a PNG image from my.blend:

blender-thumbnailer.py /input/my.blend /output/thumb.png

Details

To extract the thumbnail from a .blend file you have to write a basic .blend file parser, however - this is written in such a way that its quite simple to extract the data without having to understand the entire blend file format, instead - you only need to read one of the serialized BHead blocks. (See link above).

Yes, this is possible and infact has been done already,
the script to do this comes bundled with Blender: see blender-thumbnailer.py.

This Python script runs without Blender, eg extracting a PNG image from my.blend:

blender-thumbnailer.py /input/my.blend /output/thumb.png

Details

To extract the thumbnail from a .blend file you have to use a basic .blend file parser, however - this is written in such a way that its quite simple to extract the data without having to understand the entire blend file format, instead - you only need to read one of the serialized BHead blocks. (See link above).

Using blender-thumbnailer.py as a reference: writing this in another programming language, or saving as a different image format should not be a very big task.

Yes, this is possible and infact has been done already,
the script to do this comes with blender.

This script runs without Blender, eg extracting a thumb from my.blend:

blender-thumbnailer.py /input/my.blend /output/thumb.png

Details

To extract the thumbnail from a .blend file you have to write a basic .blend file parser, however - this is written in such a way that its quite simple to extract the data without having to understand the entire blend file format, instead - you only need to read one of the serialized BHead blocks. (See link above).

Yes, this is possible and infact has been done already,
the script to do this comes bundled with Blender: see blender-thumbnailer.py.

This script runs without Blender, eg extracting a PNG image from my.blend:

blender-thumbnailer.py /input/my.blend /output/thumb.png

Details

To extract the thumbnail from a .blend file you have to write a basic .blend file parser, however - this is written in such a way that its quite simple to extract the data without having to understand the entire blend file format, instead - you only need to read one of the serialized BHead blocks. (See link above).