I vaguely remember that some basic tools existed for changing logos and text on s1mp3 players, but I don't think there was much more than that. You could spend hundreds or thousands of hours reverse engineering your players and developing new software for it, but since it's a 15 years old platform with very limited hardware, it probably makes more sense to buy something modern with a conventional CPU if you want to get into firmware development.
Regardless, no rockbox port, and anyway this is a duplicate of the older threads on the s1, so I'm closing this.