Disclaimer: I could be wrong, so please be advised.
I have been looking for this feature in the documentation and forums without success. The VLC design team is doing a great job so hopefully they can give us some positive news.
In my case, I want to stretch/distort the image to fit the display (in my case an iPod touch). Since the display is so small it would be nice to fit the display rather than have black bars at the top and bottom of the display. So technically I need to change the aspect ratio to/from a non-standard value.