I can't realistically see anyone writing something that would be a general replacement for the player software, but maybe an alternate player that works differently to suit your own needs.

I'd be interested in hearing what functionality you'd put in your own player software that isn't already covered.

If you're serious, I'd suggest checking out madplay as an mp3 decoder. It's all integer based, and has sucessfully been ported to the tivo which has a 54MHz ARM chip. Or if you wanna write your own decoder, you could start by reading this.