Safecom technologies, a manufacturer that also uses the AR7WRD platform has a very interesting instruction you might be interested in.
This company instructs it's end-users to upgrade the bootloader themselves with these commands and then flash a new firmware.
cp psbl-4-marvell.bin /dev/mtdblock/2/
Now I know this is possible, but it is the first time I see any manufacturer instructing it's users to do this.
Link to this instructions
http://safecom.cn/code/sub/driversmanua … subcatid=1
download PC TOOLS and iside you will find the instructions in PDF format.