With the Nano clones the battery is around 130mAh and battery space is very limited so finding a replacement that will fit is a problem.
I have replaced the batteries in 3 players with iPod replacement batteries but none of them were nano style players. One of them managed to play for 18 hours with a 850mAh replacement.
http://www.mympxplayer.org/18-hours-...ge-vt2412.html
You could check the ipod shuffle repacement batteries size wise they are about 360mAh which would almost triple your playtime.
The only other option would be a usb battery pack so you could charge the player on the go. I have a 4xAA one that gives 60 hours continuous playback.
http://www.mympxplayer.org/external-...ck-vt2435.html