i do not develope rockbox, but i do not think that you can overcharge while using it, i do belive that charging is handled by the digital audio player (dap) itself and is not affected by rockbox, when your player is fully charged the amount of power given to the player through the charging cable (usb, A.C or D.C) should only be enough to keep the player on and should bypass the battery completly when it is fully charged.
But i could be completly wrong, you could try doin a search to see if you can find yor answer though