Why should we overwrite the firmware if we can already achieve all the things rockbox features with the current firmware?
Where do you get this idea of overwriting the GP2X's firmware from? Rockbox runs alongside the retail OSes of the iRiver and iPod models (I'm not 100% sure of the Archos' and the X5 is on its way to dual-boot), so that is clearly one way to approach it.
FWIW, if it's achievable (as it seems), you'd be best porting it as an app within the GP2X Firmware, rather than a separate firmware as with the other Rockbox targets ...