proportional horizontal space in accelerando and ritardando

[Caio Giovaneti de Barros]

Here's a problem more or less common in contemporary music notation (and 
very common in contemporary me, right now)

In some pieces of music it's common to reflect accelerandos and 
ritardandos by proportionally increasing and decreasing (respectively) 
the distance between notes.

Here is an example, after a quick search:

http://www.music.indiana.edu/departments/academic/composition/style-guide/images/newimages/featherEX.gif

What do you think would be the best way to implement this in Lilypond?