The Rockbox Development team has had some success booting their Rockbox firmware for the MR-100. Here's a little info on what Rockbox is, straight from their website.

Rockbox is an open source firmware replacement for a growing number of MP3 players. Rockbox aims to be considerably more functional and efficient than your device's stock firmware while remaining easy to use and customizable. Rockbox is written by users, for users.

Here is a helloworld program Lowlight of the Rockbox dev team, you can also see the beginnings of an interface :

Cheers to homielowe for the headsup, get behind this project and read more about it in this thread.